drm/i915: Disable power asynchronously during DP AUX transfers

In a follow-up patch we will restrict holding the reference on the AUX
power domain to the AUX transfer function. To avoid the unnecessary
on-off-on power togglings drop the reference asynchronously.

There is no reason we couldn't do this in general and also put the
reference asynchronously in pps_unlock(); but that's a separate change
that can be done as a follow-up.

Cc: Ville Syrjala <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/20190509173446.31095-6-imre.deak@intel.com