nouveau: fix number of surfaces in video buffer, use defines

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nouveau: set buffer format of video buffer

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv30: hook up PMPEG support via nouveau_video, enables XvMC to work

Force the format to be the reasonable format that doesn't require an
inverse z-scan.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50: allow forcing PMPEG use, for ease of testing

This also allows people who don't want to install the binary blobs
required for VP2 to still get MPEG decoding.

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