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