backend-drm: Don't take buffer-release reference for cursor views

We just copy the SHM buffer straight into a separately-allocated GBM BO,
so no need to take a reference on the buffer itself or keep it from
being released.

All drm_output_try_view_on_plane really does at this point is to call
the prepare_*_view function for the requisite plane type, and take a ref
on the weston_buffer from the client. Given that we don't need to keep
the client buffer alive, we can short-circuit
drm_output_try_view_on_plane, and instead just call
drm_output_prepare_cursor_view directly when we have a cursor plane.

This also makes it easier to just remove drm_output_try_view_on_plane in
following patches when we merge the overlay/scanout plane path into one.
Doing so gives us two clearly-separated paths: one for copying a SHM
client buffer into a cursor, and another for directly scanning out
client content.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/state-propose.c

@@ -348,9 +348,6 @@ drm_output_try_view_on_plane(struct drm_plane *plane,
 	       zpos != DRM_PLANE_ZPOS_INVALID_PLANE);
 
 	switch (plane->type) {
-	case WDRM_PLANE_TYPE_CURSOR:
-		ps = drm_output_prepare_cursor_view(state, ev, zpos);
-		break;
 	case WDRM_PLANE_TYPE_OVERLAY:
 		ps = drm_output_prepare_overlay_view(plane, state, ev, mode,
 						     fb, zpos);
@@ -680,8 +677,12 @@ drm_output_prepare_plane_view(struct drm_output_state *state,
 			     "from candidate list, type: %s\n",
 			     plane->plane_id, p_name);
 
+		if (plane->type == WDRM_PLANE_TYPE_CURSOR) {
+			ps = drm_output_prepare_cursor_view(state, ev, zpos);
+		} else {
 			ps = drm_output_try_view_on_plane(plane, state, ev,
 							  mode, fb, zpos);
+		}
 		if (ps) {
 			drm_debug(b, "\t\t\t\t[view] view %p has been placed to "
 				     "%s plane with computed zpos %"PRIu64"\n",