drm/nouveau/nv50/disp: Fix modeset on G94

Commit 1dce6264

 introduced a regression
spotted on several G94 (FDObz #85160). This device seems to expect the
vblank period to be set after setting scale instead of before.

V2: shove this in a separate function

This is a candidate bug-fix for 3.18

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Tested-by: Zlatko Calusic <zcalusic@bitsync.net>
Tested-by: Michael Riesch <michael@riesch.at>
Tested-by: "poma" <pomidorabelisima@gmail.com>
Tested-by: Adam Williamson <adamw@happyassassin.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>