drm/i915: Account for space on the ring buffer consumed whilst wrapping.

If we fill the tail of the physical ring buffer with NOOP when wrapping,
we need to account for the reduction in available space.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -687,6 +687,7 @@ int intel_wrap_ring_buffer(struct drm_device *dev,
 		*virt++ = MI_NOOP;
 
 	ring->tail = 0;
+	ring->space = ring->head - 8;
 
 	return 0;
 }