drm/vblank: Document drm_crtc_vblank_restore constraints

I got real badly confused when trying to review a fix from Ville for
this. Let's try to document better what's required for this, and check
the minimal settings at runtime - we can't check ofc that there's
indeed no races in the driver callback.

Also noticed that the drm_vblank_restore version is unused, so lets
unexport that while at it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209101523.2954281-1-daniel.vetter@ffwll.ch