drm/nv50/kms: Set VBLANK time in modeset script

Solves blinking on reclocking memory. The value set is an underestimate, but
with non-reduced vblanking this should give us plenty of time

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>