drm/i915: Check C for null pointer rather than B

There are two occasions where pointer B is being check for a NULL
when it should be pointer C instead. Fix these.

Detected by CoverityScan, CID#1436348,1436349 ("Logically Dead Code")

Fixes: 47624cc3 ("drm/i915: Import the kfence selftests for i915_sw_fence")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518133942.5660-1-colin.king@canonical.comSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/selftests/i915_sw_fence.c

@@ -311,7 +311,7 @@ static int test_AB_C(void *arg)
 	}
 
 	C = alloc_fence();
-	if (!B) {
+	if (!C) {
 		ret = -ENOMEM;
 		goto err_B;
 	}
@@ -388,7 +388,7 @@ static int test_C_AB(void *arg)
 	}
 
 	C = alloc_fence();
-	if (!B) {
+	if (!C) {
 		ret = -ENOMEM;
 		goto err_B;
 	}