drm/vkms: Add config for default plane pipeline

drm/vkms: Add config for default plane pipeline

With the introduction of color pipeline in VKMS, the default device may
have planes with color pipelines. To avoid breaking existing uAPI,
create a kernel argument to disable them by default and a vkms_config
field to configure the plane.

This field is not definitive and will be replaced once the uAPI will be
able to configure color pipelines. For now devices created with ConfigFS
will not have any color pipeline so we can decide later how the uAPI
will look like.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patch.msgid.link/20251115000237.3561250-17-alex.hung@amd.com