drm/i915/display/icl: Disable transcoder port sync as part of crtc_disable() sequence

This clears the transcoder port sync bits of the TRANS_DDI_FUNC_CTL2
register during crtc_disable().

v3:
* Rebase on maarten's patches
v2:
* Directly write the trans_port_sync reg value (Maarten)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191018172725.1338-5-manasi.d.navare@intel.com