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 <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: <[email protected]> # v4.10+
Signed-off-by: Maarten Lankhorst <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Sean Paul <[email protected]>
[mlankhorst: Use if (ret) to remove the goto in success case.]
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>