drm/i915: enable forcewake voodoo also for gen6

We still have reports of missed irqs even on Sandybridge with the
HWSTAM workaround in place. Testing by the bug reporter gets rid of
them with the forcewake voodoo and no HWSTAM writes.

Because I've slightly botched the rebasing I've left out the ACTHD
readback which is also required to get IVB working. Seems to still
work on the tester's machine, so I think we should go with the more
minmal approach on SNB. Especially since I've only found weak evidence
for holding forcewake while waiting for an interrupt to arrive, but
none for the ACTHD readback.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45181
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45332


Tested-by: Nicolas Kalkhof nkalkhof()at()web.de
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>