drm/i915: separate subplatform init and runtime feature init

Adjusting ->port_mask does not belong in
intel_device_info_subplatform_init(), but rather
intel_display_device_info_runtime_init().

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/1035800e623e13d5e7baf1215b07e7cdad5eb764.1696336887.git.jani.nikula@intel.com