drm/ast: Detect AST 1400 model

Detect the 5th-generation AST 1400. Allows to simplify the code
for widescreen support.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-12-tzimmermann@suse.de