drm: add unpin function to prime helpers

Prevents buffers from being pinned forever.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index b06f5af..57f60a7 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -930,6 +930,7 @@
 				struct dma_buf *dma_buf);
 	/* low-level interface used by drm_gem_prime_{import,export} */
 	int (*gem_prime_pin)(struct drm_gem_object *obj);
+	void (*gem_prime_unpin)(struct drm_gem_object *obj);
 	struct sg_table *(*gem_prime_get_sg_table)(struct drm_gem_object *obj);
 	struct drm_gem_object *(*gem_prime_import_sg_table)(
 				struct drm_device *dev, size_t size,