drm/ast: Initialize ASTDP in ast_post_gpu()

Remove the call to ast_dp_launch() from ast_detect_tx_chip() and
perform it unconditionally in ast_post_gpu().

Also add error handling: the detection code apparently used
ast_dp_launch() to test for a working ASTDP, falling back to VGA on
errors. As the VBIOS reports ASTDP, silently ignoring errors is
questionable behavior. With the refactoring, failing to initialize
the ASTDP will also fail probing the driver.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117103450.28692-5-tzimmermann@suse.de