drm/i915/dmc: fix i915_reg_t usage

i915_reg_t is supposed to be a somewhat opaque data type, not to be
looked inside.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b813d0cf1b800386742eb3f919b0ba8d5c182a2a.1647870374.git.jani.nikula@intel.com