drm/i915/dmc: 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/bc144ab3565b10e71244cd09f72ce7df86f4b5c6.1670433372.git.jani.nikula@intel.com