drm/i915/gmbus: stop using implicit dev_priv in register definitions

Since the beginning of time, we've implicitly assumed dev_priv is
present as a local variable in many places. We've gone a long way in
removing many of them, but the register macro definitions are the last
holdout. Remove them from the gmbus macros.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a4f482c1f523d7225420f8386f1eea6d639db843.1661855191.git.jani.nikula@intel.com