drm/i915/dp-aux: switch to intel_de_* register accessors in display code

Avoid direct uncore use in display code.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/262a0cf647b37e27a1c7776d3816e1b4ef959a91.1670433372.git.jani.nikula@intel.com