drm/i915: Update hotplug interrupts register definitions for Sandybridge

On Sandybridge, the bit definition for hotplug on SDE has changed, so
update the code to new definition.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30378


Cc: stable@kernel.org
Signed-off-by: Yuanhan Liu <yuanhan.liu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>