drm/i915/power: use intel_de_wait_custom() instead of wait_for_us()

Prefer the register read specific wait function over i915 wait_for_us().

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/154b681d9545b26453920b155656a65ce685da2a.1753956266.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>