drm/ast: Add support_fullhd flag to struct ast_device

Detect support for 1920x1080 (FullHD) 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.

For Gen3, a new branch duplicates the Gen2 logic and adds a test for
AST2200. Gen2 adds a test for AST2100.

v2:
- use fullhd flag for setting max width/height

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-6-tzimmermann@suse.de