drm/qxl: Use unlocked gem unreferencing

For drm_gem_object_unreference callers are required to hold
dev->struct_mutex, which these paths don't. Enforcing this requirement
has become a bit more strict with

commit ef4c6270bf2867e2f8032e9614d1a8cfc6c71663
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 15 09:36:25 2015 +0200

    drm/gem: Check locking in drm_gem_object_unreference

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-4-git-send-email-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-5-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/qxl/qxl_fb.c

@@ -443,11 +443,11 @@ out_unref:
 		}
 	}
 	if (fb && ret) {
-		drm_gem_object_unreference(gobj);
+		drm_gem_object_unreference_unlocked(gobj);
 		drm_framebuffer_cleanup(fb);
 		kfree(fb);
 	}
-	drm_gem_object_unreference(gobj);
+	drm_gem_object_unreference_unlocked(gobj);
 	return ret;
 }