drm/ast: Remove TX-chip bitmask

drm/ast: Remove TX-chip bitmask

The CRTC has only one output attached to it. Store the output's type
of TX chip in a single field and remove the related bitmask.

Turn the type-less output field in struct ast_device into a union, as
only one of its fields will be used at a time.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240903132601.91618-1-tzimmermann@suse.de