drm/vmwgfx/vmwgfx_so: Add description for 'vmw_view's 'rcu' member

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/vmwgfx/vmwgfx_so.c:73: warning: Function parameter or member 'rcu' not described in 'vmw_view'

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Zack Rusin <zackr@vmware.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181601.3432599-4-lee.jones@linaro.org

drivers/gpu/drm/vmwgfx/vmwgfx_so.c

@@ -42,6 +42,7 @@
 /**
  * struct vmw_view - view metadata
  *
+ * @rcu: RCU callback head
  * @res: The struct vmw_resource we derive from
  * @ctx: Non-refcounted pointer to the context this view belongs to.
  * @srf: Refcounted pointer to the surface pointed to by this view.