commit 15239099 upstream.

We need it to restore the ilk rc6 context, since the gpu wait no
requires interrupts. But in general having interrupts around should
help in code sanity, since more and more stuff is interrupt driven.

This regression has been introduced in

commit 3e960501
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Nov 27 16:22:54 2012 +0000

    drm/i915: Rearrange code to only have a single method for waiting upon the ring

Like in the driver load code we need to make sure that hotplug
interrupts don't cause havoc with our modeset state, hence block them
with the existing infrastructure. Again we ignore races where we might
loose hotplug interrupts ...

Note that the driver load part of the regression has already been
fixed in

commit 52d7eced
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Dec 1 21:03:22 2012 +0100

    drm/i915: reorder setup sequence to have irqs for output setup

v2: Add a note to the commit message about which patch fixed the
driver load part of the regression. Stable kernels need to backport
both patches.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=54691
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reported-and-Tested-by: Ilya Tumaykin <itumaykin@gmail.com>
Reviewed-by: Chris wilson <chris@chris-wilson.co.uk> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>