drm/gem: dont init "ret" in drm_gem_mmap()

There is no need to initialize this variable, so drop it. Otherwise, the
compiler won't warn if we use it unintialized.

Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: David Herrmann <[e-mail]>

drivers/gpu/drm/drm_gem.c

···
 	struct drm_device *dev = priv->minor->dev;
 	struct drm_gem_object *obj;
 	struct drm_vma_offset_node *node;
-	int ret = 0;
+	int ret;
 
 	if (drm_device_is_unplugged(dev))
 		return -ENODEV;