drm/nouveau: always wait for the exclusive fence

Drivers also need to to sync to the exclusive fence when
a shared one is present.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702111642.17259-4-christian.koenig@amd.com