drm/i915: Drop forcewake w/a for missed interrupts/seqno on Sandybridge

I believe, and an evening of i-g-t, that our original workaround for the
missed interrupts on Sandybridge, that of holding forcewake whilst we
wait for an interrupts, is no longer required. This leaves us dependent
on the second workaround of forcing an UC read of the ACTHD before
reading back the seqno from the snooped HWS. Dropping the forcewake
should allow us to conserve a little power, not much as the GPU is meant
to be busy whilst we wait for it!

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>