drm/i915: add gmch substruct to struct drm_i915_private

Group the GMCH related members together.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c10c8f16cb5d12041e009f788bd9810225d6962d.1673958757.git.jani.nikula@intel.com