drm/i915/ringbuffer: Idling requires waiting for the ring to be empty

...which is measured by the size and not the amount of space remaining.

Waiting upon size-8, did one of two things. In the common case with more
than 8 bytes available to write into the ring, it would return
immediately. Otherwise, it would timeout given the impossible condition
of waiting for more space than is available in the ring, leading to
warnings such as:

[drm:intel_cleanup_ring_buffer] *ERROR* failed to quiesce render ring
whilst cleaning up: -16
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -165,7 +165,7 @@ void intel_cleanup_ring_buffer(struct intel_ring_buffer *ring);
 int __must_check intel_wait_ring_buffer(struct intel_ring_buffer *ring, int n);
 static inline int intel_wait_ring_idle(struct intel_ring_buffer *ring)
 {
-	return intel_wait_ring_buffer(ring, ring->space - 8);
+	return intel_wait_ring_buffer(ring, ring->size - 8);
 }
 
 int __must_check intel_ring_begin(struct intel_ring_buffer *ring, int n);