drm/i915: Kick the power sequencer before AUX transactions

When we pick a new power sequencer for the port but we're not doing a
full modeset, the power sequencer may have locked on to another port (or
no port). So kick it a bit to make sure it controls the port we want.

Again just like when we attempt to actually enable the DP port, we
must first write the port register with the approriate value except
the enable bit, and then we must enable the port to make the power
sequencer happy. In this case since we don't want the port actually
enabled we just toggle it on and immediately back off. Going forward
the power sequencer will keep working on that specific port until again
moved to another port.

v2: Refine the kick procedure

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>