drm/i915: Avoid race of intel_crt_detect_hotplug() with HPD interrupt, v2

drm/i915: Avoid race of intel_crt_detect_hotplug() with HPD interrupt, v2

An HPD interrupt may fire while we are in a function that changes
the PORT_HOTPLUG_EN register - especially when an HPD interrupt
storm occurs.
Since the interrupt handler changes the enabled HPD lines when it
detects such a storm the read-modify-write cycles may interfere.
To avoid this, shiled the rmw cycles with IRQ save spinlocks.

Changes since v1:
- Implement a function which takes care of accessing PORT_HOTPLUG_EN.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>