drm/nouveau: Fix error handling in nv50_disp_atomic_commit

drm/nouveau: Fix error handling in nv50_disp_atomic_commit

Make it more clear that post commit return ret is really return 0,

and add a missing drm_atomic_helper_cleanup_planes when
drm_atomic_helper_wait_for_fences fails.

Fixes: 839ca903f12e ("drm/nouveau/kms/nv50: transition to atomic interfaces internally")
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170711143314.2148-2-maarten.lankhorst@linux.intel.com
Reviewed-by: Sean Paul <seanpaul@chromium.org>
[mlankhorst: Use if (ret) to remove the goto in success case.]
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>