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

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

Commit 1dce6264045cd23e9c07574ed0bb31c7dce9354f 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 <[email protected]>
Tested-by: Zlatko Calusic <[email protected]>
Tested-by: Michael Riesch <[email protected]>
Tested-by: "poma" <[email protected]>
Tested-by: Adam Williamson <[email protected]>
Signed-off-by: Ben Skeggs <[email protected]>