drm/i915/dp_mst: Clear the ACT sent flag during encoder disabling too

During encoder enabling we clear the flag before starting the ACT
sequence and wait for the flag, but the clearing is missing during
encoder disabling, add it there too. Since nothing cleared the flag
automatically we could've run subsequent disabling steps too early.

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/20200616141855.746-5-imre.deak@intel.com