drm/nouveau: kick vram functions out into an "engine"

NVC0 will be able to share some of nv50's paths this way.  This also makes
it the card-specific vram code responsible for deciding if a given set
of tile_flags is valid, rather than duplicating the allowed types in
nv50_vram.c and nouveau_gem.c

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>