Clean up i915_reg.h.

v2: Drop a redundant comment (Ville)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/679b7395a78c53006ac07448706f1809b74810de.1714128645.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>