drm/i915: Move gmbus definitions out of i915_reg.h

They're not related to registers, so move them to the more appropriate
intel_gmbus.h

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816012343.36433-4-daniele.ceraolospurio@intel.com