drm/i915: make PCH_GMBUS* definitions private to gvt

This is the only place that they are being used - the others use the
GMBUS* macros that rely on dev_priv being already properly initialized.

Cc: intel-gvt-dev@lists.freedesktop.org
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727193647.8639-1-lucas.demarchi@intel.com