drm/exynos: drop struct_mutex from exynos_drm_gem_get_ioctl

The only things this protects is reading ->flags and ->size, both of
which are invariant over the lifetime of an exynos gem bo. So no
locking needed at all (besides that, nothing protects the writers
anyway).

Aside: exynos_gem_obj->size is redundant with
exynos_gem_obj->base.size and probably should be removed.

v2: Use _unlocked unreference (Daniel Stone).

Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-9-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/exynos/exynos_drm_gem.c

@@ -362,12 +362,9 @@ int exynos_drm_gem_get_ioctl(struct drm_device *dev, void *data,
 	struct drm_exynos_gem_info *args = data;
 	struct drm_gem_object *obj;
 
-	mutex_lock(&dev->struct_mutex);
-
 	obj = drm_gem_object_lookup(dev, file_priv, args->handle);
 	if (!obj) {
 		DRM_ERROR("failed to lookup gem object.\n");
-		mutex_unlock(&dev->struct_mutex);
 		return -EINVAL;
 	}
 
@@ -376,8 +373,7 @@ int exynos_drm_gem_get_ioctl(struct drm_device *dev, void *data,
 	args->flags = exynos_gem->flags;
 	args->size = exynos_gem->size;
 
-	drm_gem_object_unreference(obj);
-	mutex_unlock(&dev->struct_mutex);
+	drm_gem_object_unreference_unlocked(obj);
 
 	return 0;
 }