# lspci -s 1:0.0 -nn
01:00.0 VGA compatible controller [0300]: nVidia Corporation G96 [Quadro FX 380] [10de:0658] (rev a1)

# rpm -qa |grep nouve
xorg-x11-drv-nouveau-0.0.16-14.20101010git8c8f15c.fc14.x86_64

$ xvinfo 
X-Video Extension version 2.2
screen #0
  Adaptor #0: "Nouveau GeForce 8/9 Textured Video"
    number of ports: 32
    port base: 63
    operations supported: PutImage 
    supported visuals:
      depth 24, visualID 0x21
      depth 24, visualID 0x22
    number of attributes: 2
      "XV_SET_DEFAULTS" (range 0 to 0)
              client settable attribute
      "XV_SYNC_TO_VBLANK" (range 0 to 1)
              client settable attribute
              client gettable attribute (current value is 1)
    maximum XvImage size: 4096 x 4096
    Number of image formats: 4
      id: 0x32315659 (YV12)
        guid: 59563132-0000-0010-8000-00aa00389b71
        bits per pixel: 12
        number of planes: 3
        type: YUV (planar)
      id: 0x30323449 (I420)
        guid: 49343230-0000-0010-8000-00aa00389b71
        bits per pixel: 12
        number of planes: 3
        type: YUV (planar)
      id: 0x32595559 (YUY2)
        guid: 59555932-0000-0010-8000-00aa00389b71
        bits per pixel: 16
        number of planes: 1
        type: YUV (packed)
      id: 0x59565955 (UYVY)
        guid: 55595659-0000-0010-8000-00aa00389b71
        bits per pixel: 16
        number of planes: 1
        type: YUV (packed)

See, there is no XV_BRIGHTNESS or XV_CONTRAST attribute, thus no video player (or xattr tool) can set brightness or contrast on XVideo adaptor. I doubt it's unsupported by hardware. It seems to be unimplemented in nouveau driver. (I don't know whether it worked before or did not.)