drm/i915: disable interrupts when enabling PC8

The plan is to merge PC8 and D3 into a single feature, and when we're
in D3 we won't get any hotplug interrupt anyway, so leaving them
enable doesn't make sense, and it also brings us a problem. The
problem is that we get a hotplug interrupt right when we we wake up
from D3, when we're still waking up everything. If we fully disable
interrupts we won't get this hotplug interrupt, so we won't have
problems.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>