drm/i915: Fix use of invalid array size for ring->sync_seqno

There are I915_NUM_RINGS-1 inter-ring synchronisation counters, but we
were clearing I915_NUM_RINGS of them. Oops.

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Tested-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 3dfc848..812b97b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1857,7 +1857,7 @@
 
 	seqno = ring->get_seqno(ring);
 
-	for (i = 0; i < I915_NUM_RINGS; i++)
+	for (i = 0; i < ARRAY_SIZE(ring->sync_seqno); i++)
 		if (seqno >= ring->sync_seqno[i])
 			ring->sync_seqno[i] = 0;