drm/probe-helper: don't lose hotplug event

There's a race window (small for hpd, 10s large for polled outputs)
where userspace could sneak in with an unrelated connnector probe
ioctl call and eat the hotplug event (since neither the hpd nor the
poll code see a state change).

To avoid this, check whether the connector state changes in all other
->detect calls (in the current helper code that's only probe_single)
and if that's the case, fire off a hotplug event. Note that we can't
directly call the hotplug event handler, since that expects that no
locks are held (due to reentrancy with the fb code to update the kms
console).

Also, this requires that drivers using the probe_single helper
function set up the poll work. All current drivers do that already,
and with the reworked hpd handling there'll be no downside to
unconditionally setting up the poll work any more.

v2: Review from Rob Clark
- Don't bail out of the output poll work immediately if it's disabled
  to make sure we deliver the delayed hoptplug events. Instead just
  jump to the tail.
- Don't scheduel the work when it's not set up. Would be a driver bug
  since using probe helpers for anything dynamic without them
  initialized makes them all noops.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>