drm/gma500: 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.

Cc: Patrik Jakobsson <[e-mail]>
Acked-by: Patrik Jakobsson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/gma500/gem.c

···
 	int ret = 0;
 	struct drm_gem_object *obj;
 
-	mutex_lock(&dev->struct_mutex);
-
 	/* GEM does all our handle to object mapping */
 	obj = drm_gem_object_lookup(dev, file, handle);
-	if (obj == NULL) {
-		ret = -ENOENT;
-		goto unlock;
-	}
-	/* What validation is needed here ? */
+	if (obj == NULL)
+		return -ENOENT;
 
 	/* Make it mmapable */
 	ret = drm_gem_create_mmap_offset(obj);
···
 		goto out;
 	*offset = drm_vma_node_offset_addr(&obj->vma_node);
 out:
-	drm_gem_object_unreference(obj);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
+	drm_gem_object_unreference_unlocked(obj);
 	return ret;
 }