drm/i915: checking for NULL instead of IS_ERR() in mock selftests

i915_gem_request_alloc() uses error pointers.  It never returns NULLs.

Fixes: 0daf011 ("drm/i915: Mock infrastructure for request emission")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170413195217.GA26108@mwanda
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit be02f75)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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

@@ -35,7 +35,7 @@ mock_request(struct intel_engine_cs *engine,
 
 	/* NB the i915->requests slab cache is enlarged to fit mock_request */
 	request = i915_gem_request_alloc(engine, context);
-	if (!request)
+	if (IS_ERR(request))
 		return NULL;
 
 	mock = container_of(request, typeof(*mock), base);