drm: Unexport drm_plane_helper_check_update

It's for legacy drivers only (atomic ones should use
drm_atomic_helper_check_plane_state() instead), and there's no users
left except the one in the primary plane helpers.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-18-daniel.vetter@ffwll.ch