drm/ast: Warn about unsupported TX chips

A number of TX chips are listed in VGACRD1, but not supported by
the ast driver. Whether any existing product uses such a chip is
unknown. Warn if the driver encounters any. We can then add
support as necessary.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Suggested-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240916082920.56234-4-tzimmermann@suse.de