drm/i915/dsi: 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/1fe3d5ac314dd644573e9f59941e4c7f1d57b05d.1753956266.git.jani.nikula@intel.com


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