Before testing if the panel VDD is enabled on eDP cancel any pending
disable worker. This makes sure the worker will be triggered with a
delay from the last time edp_panel_vdd_schedule_off() is called, not
the first time. This avoids unnecessary overhead.

https://bugs.freedesktop.org/show_bug.cgi?id=86201

v2: use cancel_delayed_work() instead of cancel_delayed_work_sync()
as the pps_mutexes will provide the required serialization with
edp_panel_vdd_work() while the sync variant may deadlock. Suggested
by Ville Syrjälä <ville.syrjala@linux.intel.com>.
Made commit message a bit clearer.
Signed-off-by: NEgbert Eich <eich@suse.de>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>