drm/nv50: fill in crtc mode struct members from crtc_mode_fixup

The DRM uses the adjusted mode to calculate constants for vblank
timestamping.  Our encoder mode_fixup (usually) replaces this data
with our backend mode information, which doesn't have the needed
data filled in already.

Reported-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>