drm: Introduce drm_plane_reset_state() helper function

Introduce a helper function to reset the current state of a drm_plane.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>

libweston/backend-drm/drm-gbm.c

@@ -260,10 +260,7 @@ drm_output_fini_egl(struct drm_output *output)
 	if (!b->shutting_down &&
 	    output->scanout_plane->state_cur->fb &&
 	    output->scanout_plane->state_cur->fb->type == BUFFER_GBM_SURFACE) {
-		drm_plane_state_free(output->scanout_plane->state_cur, true);
+		drm_plane_reset_state(output->scanout_plane);
-		output->scanout_plane->state_cur =
-			drm_plane_state_alloc(NULL, output->scanout_plane);
-		output->scanout_plane->state_cur->complete = true;
 	}
 
 	gl_renderer->output_destroy(&output->base);

libweston/backend-drm/drm-internal.h

@@ -748,6 +748,8 @@ drm_plane_state_put_back(struct drm_plane_state *state);
 bool
 drm_plane_state_coords_for_view(struct drm_plane_state *state,
 				struct weston_view *ev, uint64_t zpos);
+void
+drm_plane_reset_state(struct drm_plane *plane);
 
 void
 drm_assign_planes(struct weston_output *output_base, void *repaint_data);

libweston/backend-drm/drm.c

@@ -1272,10 +1272,7 @@ drm_output_fini_pixman(struct drm_output *output)
 	if (!b->shutting_down &&
 	    output->scanout_plane->state_cur->fb &&
 	    output->scanout_plane->state_cur->fb->type == BUFFER_PIXMAN_DUMB) {
-		drm_plane_state_free(output->scanout_plane->state_cur, true);
+		drm_plane_reset_state(output->scanout_plane);
-		output->scanout_plane->state_cur =
-			drm_plane_state_alloc(NULL, output->scanout_plane);
-		output->scanout_plane->state_cur->complete = true;
 	}
 
 	pixman_renderer_output_destroy(&output->base);

libweston/backend-drm/state-helpers.c

@@ -261,6 +261,21 @@ drm_plane_state_coords_for_view(struct drm_plane_state *state,
 	return true;
 }
 
+/**
+ * Reset the current state of a DRM plane
+ *
+ * The current state will be freed and replaced by a pristine state.
+ *
+ * @param plane The plane to reset the current state of
+ */
+void
+drm_plane_reset_state(struct drm_plane *plane)
+{
+	drm_plane_state_free(plane->state_cur, true);
+	plane->state_cur = drm_plane_state_alloc(NULL, plane);
+	plane->state_cur->complete = true;
+}
+
 /**
  * Return a plane state from a drm_output_state.
  */