drm/i915: Shut down PCH interrupts during irq_uninstall

This masks out all interrupts and ack's any pending ones at IRQ
uninstall time to make sure we don't receive any unexpected interrupts
later on.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>