drm/atomic-helper: WARN if legacy plane fb pointers are bogus when committing duplicated state

drm_atomic_helper_commit_duplicated_state() should only be called
resume/reset/load_detect paths where plane->old_fb should always be
NULL and plane->fb should be equal to the new_plane_state->fb.
Assert that is indeed the case.

Cc: martin.peres@free.fr
Cc: chris@chris-wilson.co.uk
Cc: Dave Airlie <airlied@gmail.com> (v1)
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-5-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -3118,8 +3118,13 @@ int drm_atomic_helper_commit_duplicated_state(struct drm_atomic_state *state,
 
 	state->acquire_ctx = ctx;
 
-	for_each_new_plane_in_state(state, plane, new_plane_state, i)
+	for_each_new_plane_in_state(state, plane, new_plane_state, i) {
+		WARN_ON(plane->crtc != new_plane_state->crtc);
+		WARN_ON(plane->fb != new_plane_state->fb);
+		WARN_ON(plane->old_fb);
+
 		state->planes[i].old_state = plane->state;
+	}
 
 	for_each_new_crtc_in_state(state, crtc, new_crtc_state, i)
 		state->crtcs[i].old_state = crtc->state;