drm/i915: move pch_ssc_use to display sub-struct under dpll

Move the display related member to the struct drm_i915_private display
sub-struct.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230117143946.2426043-2-jani.nikula@intel.com