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: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66163


Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>