drm/armada: 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
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/gpu/drm/armada/armada_crtc.c

@@ -972,7 +972,7 @@ static void armada_drm_crtc_destroy(struct drm_crtc *crtc)
 	struct armada_private *priv = crtc->dev->dev_private;
 
 	if (dcrtc->cursor_obj)
-		drm_gem_object_unreference(&dcrtc->cursor_obj->obj);
+		drm_gem_object_unreference_unlocked(&dcrtc->cursor_obj->obj);
 
 	priv->dcrtc[dcrtc->num] = NULL;
 	drm_crtc_cleanup(&dcrtc->crtc);

drivers/gpu/drm/armada/armada_gem.c

@@ -352,13 +352,13 @@ int armada_gem_mmap_ioctl(struct drm_device *dev, void *data,
 		return -ENOENT;
 
 	if (!dobj->obj.filp) {
-		drm_gem_object_unreference(&dobj->obj);
+		drm_gem_object_unreference_unlocked(&dobj->obj);
 		return -EINVAL;
 	}
 
 	addr = vm_mmap(dobj->obj.filp, 0, args->size, PROT_READ | PROT_WRITE,
 		       MAP_SHARED, args->offset);
-	drm_gem_object_unreference(&dobj->obj);
+	drm_gem_object_unreference_unlocked(&dobj->obj);
 	if (IS_ERR_VALUE(addr))
 		return addr;