drm/i915/execlists: Avoid sync calls during park

Since we allow ourselves to use non-process context during parking, we
cannot allow ourselves to sleep and in particular cannot call
del_timer_sync() -- but we can use a plain del_timer().

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111375


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812091045.29587-1-chris@chris-wilson.co.uk