drm/i915: Implement port sync for SKL+

drm/i915: Implement port sync for SKL+

Transcoder port sync was introduced to the hardware in BDW. We
can trivially enable it for SKL+ since the same codepaths are
already used for ICL+ port sync. The only difference is the actual
location of the bits we need to poke.

We leave BDW out (at least for now) since it uses different modeset
paths that haven't been adapted for port sync, and IIRC using the
feature would involve some extra workarounds we've not implemented.

Pre-BDW hardware does not support port sync so we'd have to tweak
the modeset sequence to start the pipes as close together as possible
and hope for the best. So far no one has seriously tried to implement
that.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/27
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313164831.5980-9-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>