drm/ast: astdp: Perform link training during atomic_enable

drm/ast: astdp: Perform link training during atomic_enable

The place for link training is in the encoder's atomic_enable
helper. Remove all related tests from other helper ASTDP functions;
especially ast_astdp_is_connected(), which tests HPD status.

DP link training is controlled by the firmware. A status flag reports
success or failure. The process can be fragile on Aspeed hardware. Moving
the test from connector detection to the atomic_enable allows for several
retries and a longer timeout.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240717143319.104012-5-tzimmermann@suse.de