backend-drm: Do not overwrite plane's index when creating virtual plane

Starting with commit 4cde507be6 "backend-drm: fix plane sorting" the
plane list will have a descending order of the planes rather than ascending.

This reversed order had the side-effect of exposing the fact that we
don't set-up a plane index when creating the drm_plane using the DRM
virtual API. Without settting a plane index for that drm_plane we
effectively overwrite the plane index which has the 0 (zero) entry.

This wasn't an issue before commit 4cde507be6 "backend-drm: fix
plane sorting" as it seems we never picked up that plane index as
being a suitable one due to the fact that those were assigned to primary
planes, but after that commit, the cursor plane will be one getting
the 0 (zero) plane index.

Finally, this would trip over because we attempt to place a (cursor)
view on a primary plane (where it would've normally be a cursor
plane) and we end up with no framebuffer ref.

This is fixed trivially by assigning a plane index, different than the
ones already created by create_spirtes().

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
(cherry picked from commit 27ce9dadd8865b266f72f848b784d61aeaf8b228)

libweston/backend-drm/drm-virtual.c

@@ -81,6 +81,20 @@ drm_virtual_crtc_destroy(struct drm_crtc *crtc)
 	free(crtc);
 }
 
+static uint32_t
+get_drm_plane_index_maximum(struct drm_device *device)
+{
+	uint32_t max = 0;
+	struct drm_plane *p;
+
+	wl_list_for_each(p, &device->plane_list, link) {
+		if (p->plane_idx > max)
+			max = p->plane_idx;
+	}
+
+	return max;
+}
+
 /**
  * Create a drm_plane for virtual output
  *
@@ -125,6 +139,7 @@ drm_virtual_plane_create(struct drm_device *device, struct drm_output *output)
 		goto err;
 
 	weston_plane_init(&plane->base, b->compositor, 0, 0);
+	plane->plane_idx = get_drm_plane_index_maximum(device) + 1;
 	wl_list_insert(&device->plane_list, &plane->link);
 
 	return plane;