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

drm/i915/gmbus: add wrapper for gmbus mmio base

Don't repeat the same thing so much.

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/db360d824d47601d5ca843afa6f5d6ee17d0e514.1661855191.git.jani.nikula@intel.com

drm/i915/gmbus: whitespace cleanup in reg definitions

Simple whitespace cleanup and comment movement.

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/50df38c9f3e53dba64429b7eaa6c0d7bfaf74078.1661855191.git.jani.nikula@intel.com

drm/i915/gmbus: split out gmbus regs in a separate file

Declutter i915_reg.h, and also observe very few places need the gmbus
register defitions.

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/820807f404e548ab365b934d44f01b306eaa28c2.1661855191.git.jani.nikula@intel.com