drm_probe_helper: Cancel previous job before starting new one

Currently we schedule a call to output_poll_execute from drm_kms_helper_poll_enable for 10s in future.
Later we try to replace that in drm_helper_probe_single_connector_modes with a 0s schedule with delayed_event set.

But as there is already a job in the queue this fails, and the immediate job we wanted with delayed_event set
doesn't occur until 10s later.

And that call acts as if connector state has changed, reprobing modes.
This has a side effect of waking up a display that has been blanked.

Make sure we cancel the old job before submitting the immediate one.

Signed-off-by: Dom Cobley <popcornmix@gmail.com>