drm/i915: Take advantage of auto-polling CRT hotplug detection on PCH hardware

Both IBX and CPT have an automatic hotplug detection mode which appears to work reliably enough
that we can dispense with the manual force hotplug trigger stuff. This means that
hotplug detection is as simple as reading the current hotplug register values.

The first time the hotplug detection is activated, the code synchronously waits for a hotplug
sequence in case the hardware hasn't bothered to do a detection cycle since being initialized.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>