drm/nouveau/kms/nv50: request vblank events for commits that send completion events

This somehow fixes an issue where sync-to-vblank longer works correctly
after resume from suspend.

From a HW perspective, we don't need the IRQs turned on to be able to
detect flip completion, so it's assumed that this is required for the
voodoo in the core DRM vblank core.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>