drm/i915/display: move restore state and ctx under display sub-struct

Move display suspend/resume and display reset modeset state and ctx
members under drm_i915_private display sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221109144209.3624739-4-jani.nikula@intel.com