drm/i915: remove pre-atomic check from SKL update_primary_plane

The comment suggests the check was there for some non-fully-atomic
case, and I couldn't find a case where we wouldn't correctly
initialize plane_state, so remove the check.

Let's leave a WARN there just in case.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>