drm/i915/selftests: Fix an IS_ERR() vs NULL check

The live_context() function returns error pointers.  It never returns
NULL.

Fixes: 9c1477e8 ("drm/i915/selftests: Exercise adding requests to a full GGTT")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326050843.GA20038@kadam

drivers/gpu/drm/i915/selftests/i915_gem_evict.c

@@ -455,7 +455,7 @@ static int igt_evict_contexts(void *arg)
 			struct i915_gem_context *ctx;
 
 			ctx = live_context(i915, file);
-			if (!ctx)
+			if (IS_ERR(ctx))
 				break;
 
 			/* We will need some GGTT space for the rq's context */