drm/atomic: Fix potential use of state after free

The atomic helpers rely on drm_atomic_state_clear() to reset an atomic
state if a retry is needed due to the w/w mutexes. The subsequent calls
to drm_atomic_get_{crtc,plane,...}_state() would then return the stale
pointers in state->{crtc,plane,...}_states.
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic.c

@@ -134,6 +134,7 @@ void drm_atomic_state_clear(struct drm_atomic_state *state)
 
 		connector->funcs->atomic_destroy_state(connector,
 						       state->connector_states[i]);
+		state->connector_states[i] = NULL;
 	}
 
 	for (i = 0; i < config->num_crtc; i++) {
@@ -144,6 +145,7 @@ void drm_atomic_state_clear(struct drm_atomic_state *state)
 
 		crtc->funcs->atomic_destroy_state(crtc,
 						  state->crtc_states[i]);
+		state->crtc_states[i] = NULL;
 	}
 
 	for (i = 0; i < config->num_total_plane; i++) {
@@ -154,6 +156,7 @@ void drm_atomic_state_clear(struct drm_atomic_state *state)
 
 		plane->funcs->atomic_destroy_state(plane,
 						   state->plane_states[i]);
+		state->plane_states[i] = NULL;
 	}
 }
 EXPORT_SYMBOL(drm_atomic_state_clear);