Bug 1566165

List of loaded kernel sound modules

lsmod | grep -i snd_hda

snd_hda_codec_realtek   106496  1
snd_hda_codec_generic    86016  1 snd_hda_codec_realtek
snd_hda_codec_hdmi     57344  1
snd_hda_intel          45056  10
snd_hda_codec         151552  4 snd_hda_intel,snd_hda_codec_hdmi,snd_hda_codec_generic,snd_hda_codec_realtek
snd_hda_core           94208  5 snd_hda_intel,snd_hda_codec,snd_hda_codec_hdmi,snd_hda_codec_generic,snd_hda_codec_realtek
snd_hwdep              16384  1 snd_hda_codec
snd_pcm               118784  7 snd_hda_intel,snd_hda_codec,snd_pcm_dmaengine,snd_hda_core,snd_soc_rt5640,snd_hda_codec_hdmi,snd_soc_core
snd                    94208  32 snd_compress,snd_hda_intel,snd_hwdep,snd_seq,snd_hda_codec,snd_timer,snd_hda_codec_hdmi,snd_hda_codec_generic,snd_seq_device,snd_hda_codec_realtek,snd_soc_core,snd_pcm

p.s. The HDA over DP works fine in Windows10 with this GPU

List of ALSA devices 

$ aplay -L
null
    Discard all samples (playback) or generate zero samples (capture)
pulse
    PulseAudio Sound Server
default
    Default ALSA Output (currently PulseAudio Sound Server)
sysdefault:CARD=PCH
    HDA Intel PCH, ALC892 Analog
    Default Audio Device
front:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    Front speakers
surround21:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    2.1 Surround output to Front and Subwoofer speakers
surround40:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    4.0 Surround output to Front and Rear speakers
surround41:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Analog
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
iec958:CARD=PCH,DEV=0
    HDA Intel PCH, ALC892 Digital
    IEC958 (S/PDIF) Digital Audio Output
hdmi:CARD=HDMI,DEV=0
    HDA ATI HDMI, HDMI 0
    HDMI Audio Output
hdmi:CARD=HDMI,DEV=1
    HDA ATI HDMI, HDMI 1
    HDMI Audio Output
hdmi:CARD=HDMI,DEV=2
    HDA ATI HDMI, HDMI 2
    HDMI Audio Output
hdmi:CARD=HDMI,DEV=3
    HDA ATI HDMI, HDMI 3
    HDMI Audio Output
hdmi:CARD=HDMI,DEV=4
    HDA ATI HDMI, HDMI 4
    HDMI Audio Output

trying speaker test using

$ speaker-test -c 2 -D "sysdefault:CARD=PCH"

does produce white noise on the analogue line outputs, but attempting to test the display port audio with

$ speaker-test -c 2 -D "hdmi:CARD=HDMI,DEV=0"

for each of DEV=0 to DEV=4 produces no sound through the monitor

I had to swap from an rx460 to an rx550 due to hardware issues, the problem persisted, but after a little reading up I realised the amdgpu driver module needs the "dc=1" option to allow HD audio, I did this via 

$ cat /etc/modprobe.d/amdgpu.conf 
options amdgpu dc=1

Others may prefer to add amdgpu.dc=1 to the kernel boot command line, the audio device does now show up, and plays sound, I gather this may still be experimental, so will report back if it causes problems ...

Using the Fedora test day iso with kernel-4.17.0-200.fc28.x86_64
I can confirm that amdgpu driver now recognises the HDMI/DP audio without any modprobe or kernel command line tricks