drm/ast: Only warn about unsupported TX chips on Gen4 and later

Only Gen4 and later read the installed TX chip from the SoC. So only
warn on those generations about unsupported chips.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117103450.28692-9-tzimmermann@suse.de