drm/qxl: remove set but not used variable 'bo_old'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/qxl/qxl_display.c: In function 'qxl_primary_atomic_update':
drivers/gpu/drm/qxl/qxl_display.c:538:17: warning:
 variable 'bo_old' set but not used [-Wunused-but-set-variable]

It's not used any more after 4979904c ("drm/qxl: use shadow bo directly")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190218085459.196470-1-yuehaibing@huawei.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/qxl/qxl_display.c

@@ -535,7 +535,7 @@ static void qxl_primary_atomic_update(struct drm_plane *plane,
 {
 	struct qxl_device *qdev = plane->dev->dev_private;
 	struct qxl_bo *bo = gem_to_qxl_bo(plane->state->fb->obj[0]);
-	struct qxl_bo *bo_old, *primary;
+	struct qxl_bo *primary;
 	struct drm_clip_rect norect = {
 	    .x1 = 0,
 	    .y1 = 0,
@@ -544,12 +544,6 @@ static void qxl_primary_atomic_update(struct drm_plane *plane,
 	};
 	uint32_t dumb_shadow_offset = 0;
 
-	if (old_state->fb) {
-		bo_old = gem_to_qxl_bo(old_state->fb->obj[0]);
-	} else {
-		bo_old = NULL;
-	}
-
 	primary = bo->shadow ? bo->shadow : bo;
 
 	if (!primary->is_primary) {