drm/i915: Simplify do_idling() (Ironlake vt-d w/a)

Now that we pass along the expected interruptible nature for the
wait-for-idle, we do not need to modify the global
i915->mm.interruptible for this single call. (Only the immediate call to
i915_gem_wait_for_idle() takes the interruptible status as the other
action, dma_map_sg(), is independent of i915.ko)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470388464-28458-7-git-send-email-chris@chris-wilson.co.uk