drm/i915/ringbuffer: Fix sign of ring space.

As we presume space is signed when computing and looking for wrap along,
make it so.
Reported-by: Owain G. Ainsworth <zerooa@googlemail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -31,7 +31,7 @@ struct  intel_ring_buffer {
 
 	unsigned int	head;
 	unsigned int	tail;
-	unsigned int	space;
+	int		space;
 	struct intel_hw_status_page status_page;
 
 	u32		irq_gem_seqno;		/* last seq seem at irq time */