MAINTAINERS: Add Thomas as reviewer for ast, mgag200 and udl

I'm adding myself as reviewer for ast, mgag200 and udl. I've already
been keeping these drivers in shape for a while.

While at it I'm also setting the list and tree for ast and mgag200,
and update each driver's status to Supported. Working on these drivers
is part of my job.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915071708.4743-1-tzimmermann@suse.de