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