Add MetaKmsCrtc to represent a CRTC on the associated device. Change
MetaCrtcKms to use the ones discovered by the KMS abstraction. It still
reads the resources handed over by MetaGpuKms, but eventually it will
use only MetaKmsCrtc.
MetaKmsCrtc is a type of object that is usable both from an impl task
and from outside. All the API exposed via the non-private header is
expected to be accessible from outside of the meta-kms namespace.
https://gitlab.gnome.org/GNOME/mutter/issues/548https://gitlab.gnome.org/GNOME/mutter/merge_requests/525
