drm/tegra: Use drm_gem_object_unreference_unlocked()

This only grabs the mutex when really needed, but still has a might-
acquire lockdep check to make sure that's always possible. With this
patch Tegra DRM is officially struct_mutex free, yay!

v2: refernce_unlocked doesn't exist as kbuild spotted.

Cc: Thierry Reding <[e-mail]>
Acked-by: Thierry Reding <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
[[e-mail]: remove unused variables]
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/drm.c

···
 	if (!gem)
 		return NULL;
 
-	mutex_lock(&drm->struct_mutex);
-	drm_gem_object_unreference(gem);
-	mutex_unlock(&drm->struct_mutex);
+	drm_gem_object_unreference_unlocked(gem);
 
 	bo = to_tegra_bo(gem);
 	return &bo->base;

drivers/gpu/drm/tegra/gem.c

···
 static void tegra_bo_put(struct host1x_bo *bo)
 {
 	struct tegra_bo *obj = host1x_to_tegra_bo(bo);
-	struct drm_device *drm = obj->gem.dev;
 
-	mutex_lock(&drm->struct_mutex);
-	drm_gem_object_unreference(&obj->gem);
-	mutex_unlock(&drm->struct_mutex);
+	drm_gem_object_unreference_unlocked(&obj->gem);
 }
 
 static dma_addr_t tegra_bo_pin(struct host1x_bo *bo, struct sg_table **sgt)
···
 static struct host1x_bo *tegra_bo_get(struct host1x_bo *bo)
 {
 	struct tegra_bo *obj = host1x_to_tegra_bo(bo);
-	struct drm_device *drm = obj->gem.dev;
 
-	mutex_lock(&drm->struct_mutex);
 	drm_gem_object_reference(&obj->gem);
-	mutex_unlock(&drm->struct_mutex);
 
 	return bo;
 }