drm/ast: Move connector mode_valid function to CRTC

The tests in ast_mode_valid() verify the correct resolution for the
supplied mode. This is a limitation of the CRTC, so move the function
to the CRTC helpers. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-3-tzimmermann@suse.de