drm: Return EINVAL on duplicate objects in execbuffer object list

If userspace passes an object list with the same object appearing more
than once, we end up hitting the BUG_ON() in
i915_gem_object_set_to_gpu_domain() as it gets called a second time
for the same object.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_drv.h

@@ -457,6 +457,12 @@ struct drm_i915_gem_object {
 
 	/** for phy allocated objects */
 	struct drm_i915_gem_phys_object *phys_obj;
+
+	/**
+	 * Used for checking the object doesn't appear more than once
+	 * in an execbuffer object list.
+	 */
+	int in_execbuffer;
 };
 
 /**