drm/i915: Silence compiler warning for seltests/i915_gem_coherency

In general, the compiler should not be able to detect if we do any
passes through the test loops:

	In file included from drivers/gpu/drm/i915/i915_gem.c:5029:
	drivers/gpu/drm/i915/selftests/i915_gem_coherency.c: In function 'igt_gem_coherency':
	drivers/gpu/drm/i915/selftests/i915_gem_coherency.c:274: error: 'err' may be used uninitialized in this function
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170214143509.15719-1-chris@chris-wilson.co.ukReviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/selftests/i915_gem_coherency.c

@@ -272,7 +272,7 @@ static int igt_gem_coherency(void *arg)
 	struct drm_i915_gem_object *obj;
 	unsigned long count, n;
 	u32 *offsets, *values;
-	int err;
+	int err = 0;
 
 	/* We repeatedly write, overwrite and read from a sequence of
 	 * cachelines in order to try and detect incoherency (unflushed writes