drm/msm: 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 ef4c6270
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: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/msm_fbdev.c

@@ -121,7 +121,7 @@ static int msm_fbdev_create(struct drm_fb_helper *helper,
 		/* note: if fb creation failed, we can't rely on fb destroy
 		 * to unref the bo:
 		 */
-		drm_gem_object_unreference(fbdev->bo);
+		drm_gem_object_unreference_unlocked(fbdev->bo);
 		ret = PTR_ERR(fb);
 		goto fail;
 	}