drm/i915/dp_mst: Move clearing the ACT sent flag closer to its polling

drm/i915/dp_mst: Move clearing the ACT sent flag closer to its polling

During transcoder enabling we'll configure the transcoder in MST mode
and enable the VC payload allocation, which will start the ACT sequence.
Before waiting for the ACT sequence completion, we need to clear the ACT
sent flag, but based on the above we can do this right before enabling
the transcoder.

For clarity, move the flag clearing closer to where we wait for it.

While at it also factor out some common code.

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/20200616141855.746-3-imre.deak@intel.com