drm/vkms: Allow to configure multiple planes via configfs

drm/vkms: Allow to configure multiple planes via configfs

Create a default subgroup at /config/vkms/planes to allow to create as
many planes as required.

Tested-by: Mark Yacoub <markyacoub@google.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Co-developed-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://lore.kernel.org/r/20251016175618.10051-4-jose.exposito89@gmail.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>