drm/i915: Only clear write-domains after a successful wait-seqno

In the introduction of the non-blocking wait, I cut'n'pasted the wait
completion code from normal locked path. Unfortunately, this neglected
that the normal path returned early if the wait returned early. The
result is that read-only waits may return whilst the GPU is still
writing to the bo.

Fixes regression from
commit 3236f57a [v3.7]
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Aug 24 09:35:09 2012 +0100

    drm/i915: Use a non-blocking wait for set-to-domain ioctl
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66163
Cc: stable@vger.kernel.org
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1160,7 +1160,8 @@ i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
 	/* Manually manage the write flush as we may have not yet
 	 * retired the buffer.
 	 */
-	if (obj->last_write_seqno &&
+	if (ret == 0 &&
+	    obj->last_write_seqno &&
 	    i915_seqno_passed(seqno, obj->last_write_seqno)) {
 		obj->last_write_seqno = 0;
 		obj->base.write_domain &= ~I915_GEM_GPU_DOMAINS;