LinuxKPI-based DRM ports: move to new ports nomenclature.

After some discussions within the x11 team, it was decided to move to a new
nomenclature for the LinuxKPI-based DRM ports that correctly reflects the
FreeBSD version the ports are for. This will also allow us to move to dedicated
branches per FreeBSD release and eases porting work.

Functionally, this commit is a no-op as we are not moving to new snapshots.

Reflect the move in UPDATING and MOVED. Users using the graphics/drm-kmod meta
port should not be affected. Direct consumers of the DRM ports shall move, as
in MOVED, as follows:

drm-stable-kmod is drm-fbsd11.2-kmod
drm-next-kmod is drm-fbsd12.0-kmod
drm-devel-kmod is drm-current-kmod

Reviewed by: zeising (x11)
Approved by: zeising (x11)
Differential Revision: https://reviews.freebsd.org/D18328