drm/i915: Fix set_caching locking

On the EINVAL case we don't release struct_mutex. It should be safe to
grab the lock after checking the parameters, which also resolves the
issues.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3218,10 +3218,6 @@ int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
 	enum i915_cache_level level;
 	int ret;
 
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
-
 	switch (args->caching) {
 	case I915_CACHING_NONE:
 		level = I915_CACHE_NONE;
@@ -3233,6 +3229,10 @@ int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
 		return -EINVAL;
 	}
 
+	ret = i915_mutex_lock_interruptible(dev);
+	if (ret)
+		return ret;
+
 	obj = to_intel_bo(drm_gem_object_lookup(dev, file, args->handle));
 	if (&obj->base == NULL) {
 		ret = -ENOENT;