drm/i915/dp: use generic poll_timeout_us() instead of wait_for() in link training

drm/i915/dp: use generic poll_timeout_us() instead of wait_for() in link training

Prefer generic poll helpers over i915 custom helpers.

The functional change is losing the exponentially growing sleep of
wait_for(), which used to be 10, 20, 40, ..., 640, and 1280 us.

Use an arbitrary constant 500 us sleep instead. The timeout remains at
500 ms.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/29ab4738758fe844dc1323c4a59d5d6bdcf87308.1756383233.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>