drm/ast: Detect wide-screen support before creating modeset pipeline

Wide-screen support is relevant for mode validation. Do not detect it
before setting up the mode-setting pipeline. Gets the function call out
of the way of other initialization code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117103450.28692-2-tzimmermann@suse.de