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: 51528afe ("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

drivers/gpu/drm/i915/display/intel_ddi.c

@@ -3878,8 +3878,6 @@ static void icl_disable_transcoder_port_sync(const struct intel_crtc_state *old_
 {
 	struct intel_crtc *crtc = to_intel_crtc(old_crtc_state->uapi.crtc);
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
-	i915_reg_t reg;
-	u32 trans_ddi_func_ctl2_val;
 
 	if (old_crtc_state->master_transcoder == INVALID_TRANSCODER)
 		return;
@@ -3887,10 +3885,7 @@ static void icl_disable_transcoder_port_sync(const struct intel_crtc_state *old_
 	DRM_DEBUG_KMS("Disabling Transcoder Port Sync on Slave Transcoder %s\n",
 		      transcoder_name(old_crtc_state->cpu_transcoder));
 
-	reg = TRANS_DDI_FUNC_CTL2(old_crtc_state->cpu_transcoder);
-	trans_ddi_func_ctl2_val = ~(PORT_SYNC_MODE_ENABLE |
-				    PORT_SYNC_MODE_MASTER_SELECT_MASK);
-	I915_WRITE(reg, trans_ddi_func_ctl2_val);
+	I915_WRITE(TRANS_DDI_FUNC_CTL2(old_crtc_state->cpu_transcoder), 0);
 }
 
 static void intel_ddi_post_disable(struct intel_encoder *encoder,