drm/ast: add widescreen + rb modes from X.org driver (v2)

This syncs up the mode code from the X.org driver upstream,
and adds the mode validation step for hw that doesn't have
widescreen.

v2: (from Egbert Eich <eich@suse.de)
squash drm/ast: Use correct structure member for mode validation
to avoid bisect regression.

In struct drm_display_mode crtc_hdisplay and crtc_vdisplay are holding
the crtc parameters after mode fixup. For validation we need hdisplay and
vdisplay.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>