drm/i915: Make all plane disables use 'update_plane' (v5)

If we extend the commit_plane handlers for each plane type to be able to
handle fb=0, then we can easily implement plane disable via the
update_plane handler.  The cursor plane already works this way, and this
is the direction we need to go to integrate with the atomic plane
handler.  We can now kill off the type-specific disable functions, as
well as the redundant intel_plane_disable() (not to be confused with
intel_disable_plane()).

Note that prepare_plane_fb() only gets called as part of update_plane
when fb!=NULL (by design, to match the semantics of the atomic plane
helpers); this means that our commit_plane handlers need to handle the
frontbuffer tracking for the disable case, even though they don't handle
it for normal updates.

v2:
 - Change BUG_ON to WARN_ON (Ander/Daniel)

v3:
 - Drop unnecessary plane->crtc check since a previous patch to plane
   update ensures that plane->crtc will always be non-NULL, even for
   disable calls that might pass NULL from userspace.  (Ander)
 - Drop a s/crtc/plane->crtc/ hunk that was unnecessary.  (Ander)

v4:
 - Fix missing whitespace (Ander)

v5:
 - Use state's crtc rather than plane's crtc in
   intel_check_primary_plane().  plane->crtc could be NULL, but we've
   already fixed up state->crtc to ensure it's non-NULL (even if
   userspace passed it as NULL during a disable call).  (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>