drm/ast: Add support_wuxga flag to struct ast_device

Detect support for 1920x1200 (WUXGA) in ast_detect_widescreen(). The
flag is cleared by default. The test logic has been taken from existing
code in ast_crtc_helper_mode_valid(). The code in that function is being
replaced by the new flag.

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/20250131092257.115596-7-tzimmermann@suse.de