drm/nouveau: Handle drm_atomic_helper_swap_state failure

drm_atomic_helper_swap_state() will be changed to interruptible waiting
in the next few commits, so all drivers have to be changed to handling
failure.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Link: http://patchwork.freedesktop.org/patch/msgid/20170711143314.2148-4-maarten.lankhorst@linux.intel.com


Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>