drm/nouveau: Use drm_crtc_vblank_off/on

With atomic nv50+ is already converted over to them, but the old
display code is still using it. Found in a 2 year old patch I have
lying around to un-export these old helpers!

v2: Drop the hand-rolled versions from resume/suspend code. Now that
crtc callbacks do this, we don't need a special case for s/r anymore.

v3: Remove unused variables.

v4: Don't remove drm_crtc_vblank_off from suspend paths, non-atomic
nouveau still needs that. But still switch to drm_crtc_vblank_off
since drm_vblank_off will disappear.

Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114114101.21731-1-daniel.vetter@ffwll.ch