drm/i915/dp: Disable Port sync mode correctly on teardown

drm/i915/dp: Disable Port sync mode correctly on teardown

While clearing the Ports ync mode enable and master select bits
we need to clear the register completely instead of using disable masks

v3:
* Remove reg variable (Matt)
v2:
* Just write 0 to the reg (Ville)
* Rebase

Bugzilla: https://gitlab.freedesktop.org/drm/intel/issues/5
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Fixes: 1abf98e989d2 ("drm/i915/display/icl: Disable transcoder port sync as part of crtc_disable() sequence")
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191228031204.10189-3-manasi.d.navare@intel.com
(cherry picked from commit a3d9382bd439e7be1858abc3d5f014dd55913448)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>