drm/i915/gmch: convert intel_gmch.c to struct intel_display

drm/i915/gmch: convert intel_gmch.c to struct intel_display

Convert intel_gmch.[ch] to struct intel_display. Remove the final
dependency on struct drm_i915_private and i915_drv.h. This is in
preparation of moving the code under display/.

intel_gmch_vga_set_state() is only used internally, make it static while
at it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/a4b317656eef0781d4f53ae337083fb05ce602bc.1763578288.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>