Currently we just make sure vdd is off before suspending, but we don't
cancel the vdd off work. The work wil not touch vdd if
want_panel_vdd==false so in theory this is fine.

In the past that was perfectly fine since the vdd off work didn't do
anything when want_panel_vdd==false, so even if the work would have been
run during system resume before i915 has resumed, nothing would happen.

However since pps_lock() will now grab the power domain references before
it can check want_panel_vdd, we may end up toggling the power wells on/off
already before the driver has resumed. That is not really acceptable, so
cancel the vdd off work when suspending the encoder.

The problem appeared when pps_lock() was introduced in:
 commit 773538e8
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Thu Sep 4 14:54:56 2014 +0300

    drm/i915: Reset power sequencer pipe tracking when disp2d is off
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>