drm/vkms: Enable ARGB8888 support for writeback

The VKMS already has a function to convert ARGB16161616 to ARGB8888, so
it is possible to use this function to provide the writeback support for
the ARGB8888 format. Therefore, add the ARGB8888 format to the writeback
format list.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230515135204.115393-3-mcanal@igalia.com