drm/nouveau/pwr/memx: Make FB disable and enable explicit

Needs to be done after wait-for-VBLANK, and NVA3 requires register writes
in between.

Rather than hard-coding register writes, just split out fb_disable and
fb_enable.

v2. Squashed "fb/ramnve0: disable fb before reclocking"

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