drm/i915: Call io_schedule() whilst whilsting for the GPU

Since we are waiting upon IO completion, inform the kernel through use
of the io_schedule() call rather than the regular schedule(). This
should allow the kernel to make better decisions regarding scheduling
and power management.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1082,7 +1082,7 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
 			mod_timer(&timer, expire);
 		}
 
-		schedule();
+		io_schedule();
 
 		if (timeout)
 			timeout_jiffies = expire - jiffies;