drm/i915/dp_mst: Clear only the ACT sent flag from DP_TP_STATUS

It's not clear if the DP_TP_STATUS flags other than the ACT sent flag
have some side-effect, so don't clear those; we don't depend on the
state of these flags anyway.
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616141855.746-4-imre.deak@intel.com

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

@@ -323,7 +323,7 @@ static void clear_act_sent(struct intel_dp *intel_dp)
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 
 	intel_de_write(i915, intel_dp->regs.dp_tp_status,
-		       intel_de_read(i915, intel_dp->regs.dp_tp_status));
+		       DP_TP_STATUS_ACT_SENT);
 }
 
 static void wait_for_act_sent(struct intel_dp *intel_dp)