nv50: H.264/MPEG2 decoding support via VP2, available on NV84-NV96, NVA0

nv50: H.264/MPEG2 decoding support via VP2, available on NV84-NV96, NVA0

Adds H.264 and MPEG2 codec support via VP2, using firmware from the
blob. Acceleration is supported at the bitstream level for H.264 and
IDCT level for MPEG2.

Known issues:
 - H.264 interlaced doesn't render properly
 - H.264 shows very occasional artifacts on a small fraction of videos
 - MPEG2 + VDPAU shows frequent but small artifacts, which aren't there
   when using XvMC on the same videos

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>