drm/vkms: Add kernel-doc to the function vkms_compose_row()

The function vkms_compose_row() was introduced in the code without any
documentation. In order to make the function more clear, add a
kernel-doc to it.

Suggested-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230508220030.434118-1-mcanal@igalia.com