drm/vkms: add overlay support

Add support to overlay plane, in addition to primary and cursor
planes. In this approach, the plane composition still requires an
active primary plane and planes are composed associatively in the
order: (primary <- overlay) <- cursor

It enables to run the following IGT tests successfully:
- kms_plane_cursor:
  - pipe-A-[overlay, primary, viewport]-size-[64, 128, 256]
- kms_atomic:
  - plane-overlay-legacy
and preserves the successful execution of kms_cursor_crc,
kms_writeback and kms_flip

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/8261bf93d8a0e3ffaf81d8e7c9b3e9c229116be3.1619250933.git.melissa.srw@gmail.com