drm/i915: move gen4 GCDGMBUS save/restore to display save/restore

Logically part of the display save/restore. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910095227.9466-2-jani.nikula@intel.com