drm/i915: Add privacy-screen support (v3)

Add support for eDP panels with a built-in privacy screen using the
new drm_privacy_screen class.

Changes in v3:
- Move drm_privacy_screen_get() call to intel_ddi_init_dp_connector()

Changes in v2:
- Call drm_connector_update_privacy_screen() from
  intel_enable_ddi_dp() / intel_ddi_update_pipe_dp() instead of adding a
  for_each_new_connector_in_state() loop to intel_atomic_commit_tail()
- Move the probe-deferral check to the intel_modeset_probe_defer() helper

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005202322.700909-11-hdegoede@redhat.com