drm: Add a drm_gem_objects_lookup helper

Similar to the single handle drm_gem_object_lookup(),
drm_gem_objects_lookup() takes an array of handles and returns an array
of GEM objects.

v2:
- Take the userspace pointer directly and allocate the array.
- Expand the function documentation.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190409205427.6943-3-robh@kernel.org
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 2955aaa..5ee85c9 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -381,6 +381,8 @@ struct page **drm_gem_get_pages(struct drm_gem_object *obj);
 void drm_gem_put_pages(struct drm_gem_object *obj, struct page **pages,
 		bool dirty, bool accessed);
 
+int drm_gem_objects_lookup(struct drm_file *filp, void __user *bo_handles,
+			   int count, struct drm_gem_object ***objs_out);
 struct drm_gem_object *drm_gem_object_lookup(struct drm_file *filp, u32 handle);
 long drm_gem_reservation_object_wait(struct drm_file *filep, u32 handle,
 				    bool wait_all, unsigned long timeout);