drm/i915/dmc_wl: Use i915_mmio_reg_offset() instead of reg.reg

drm/i915/dmc_wl: Use i915_mmio_reg_offset() instead of reg.reg

The macro i915_mmio_reg_offset() is the proper interface to get a
register's offset. Use that instead of looking directly at reg.reg.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241108130218.24125-2-gustavo.sousa@intel.com