drm/ast: Reorganize widescreen test around hardware Gens

Testing for support of widescreen modes mixes up various hardware
Gens. First branch by hardware Gen, then do specific tests for each
Gen. By default, widesscreen support is disabled.

Later patches will add more specific tests for each Gen.

v2:
- move shared detection code into helper (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-5-tzimmermann@suse.de