drm/nv31/mpeg: split the nv31 and nv40 dma setting implementations

NV31 has different config bits than NV40+ do. Also fix the DMA_IMAGE
VRAM-only setting to check the right bits.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>