drm/probe-helper: Create a HPD IRQ event helper for a single connector

The drm_helper_hpd_irq_event() function is iterating over all the
connectors when an hotplug event is detected.

During that iteration, it will call each connector detect function and
figure out if its status changed.

Finally, if any connector changed, it will notify the user-space and the
clients that something changed on the DRM device.

This is supposed to be used for drivers that don't have a hotplug
interrupt for individual connectors. However, drivers that can use an
interrupt for a single connector are left in the dust and can either
reimplement the logic used during the iteration for each connector or
use that helper and iterate over all connectors all the time.

Since both are suboptimal, let's create a helper that will only perform
the status detection on a single connector.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>