drm/i915/selftests: exercise_ggtt may have nothing to do

When operating on the live_ggtt we have to find a usuable hole for our
test. It is possible for there to be no hole we can use, so initialise
the err to 0 for the early exit.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115152558.31252-1-chris@chris-wilson.co.ukReviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -958,7 +958,7 @@ static int exercise_ggtt(struct drm_i915_private *i915,
 	u64 hole_start, hole_end, last = 0;
 	struct drm_mm_node *node;
 	IGT_TIMEOUT(end_time);
-	int err = -ENODEV;
+	int err = 0;
 
 	mutex_lock(&i915->drm.struct_mutex);
 restart: