drm/ast: Move code for physical outputs into separate files

Move the modesetting code for the various transmitter chips into
their own source files before adding BMC support. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240815151953.184679-2-tzimmermann@suse.de