drm/ast: Detect AST 1300 model

Detect the 4th-generation AST 1300. 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/20230621130032.3568-11-tzimmermann@suse.de