drm/vkms: drop "Multiple overlay planes" TODO

Remove the task from the TODO list.
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220107182809.141003-3-jose.exposito89@gmail.com

Documentation/gpu/vkms.rst

@@ -124,8 +124,6 @@ Add Plane Features
 
 There's lots of plane features we could add support for:
 
-- Multiple overlay planes. [Good to get started]
-
 - Clearing primary plane: clear primary plane before plane composition (at the
   start) for correctness of pixel blend ops. It also guarantees alpha channel
   is cleared in the target buffer for stable crc. [Good to get started]