drm/exynos: Drop dev->struct_mutex from mmap offset function

Simply forgotten about this when I was doing my general cleansing of
simple gem mmap offset functions. There's nothing but core functions
called here, and they all have their own protection already.

Aside: DRM_ERROR for userspace controlled input isn't great, but
that's for another patch.

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/1459330852-27668-7-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/exynos/exynos_drm_gem.c

@@ -458,8 +458,6 @@ int exynos_drm_gem_dumb_map_offset(struct drm_file *file_priv,
 	struct drm_gem_object *obj;
 	int ret = 0;
 
-	mutex_lock(&dev->struct_mutex);
-
 	/*
 	 * get offset of memory allocated for drm framebuffer.
 	 * - this callback would be called by user application
@@ -469,16 +467,13 @@ int exynos_drm_gem_dumb_map_offset(struct drm_file *file_priv,
 	obj = drm_gem_object_lookup(dev, file_priv, handle);
 	if (!obj) {
 		DRM_ERROR("failed to lookup gem object.\n");
-		ret = -EINVAL;
-		goto unlock;
+		return -EINVAL;
 	}
 
 	*offset = drm_vma_node_offset_addr(&obj->vma_node);
 	DRM_DEBUG_KMS("offset = 0x%lx\n", (unsigned long)*offset);
 
-	drm_gem_object_unreference(obj);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
+	drm_gem_object_unreference_unlocked(obj);
 	return ret;
 }