drm/nouveau: Drop mutex_lock_nested for atomic

Purely conjecture, but I think the original locking inversion with the
legacy page flip code between flipping and ttm's bo move function
shoudn't exist anymore with atomic: With atomic the bo pinning and
actual modeset commit is completely separated in the code patsh.

This annotation was originally added in

commit 060810d7
Author: Ben Skeggs <bskeggs@redhat.com>
Date:   Mon Jul 8 14:15:51 2013 +1000

    drm/nouveau: fix locking issues in page flipping paths

due to

commit b580c9e2
Author: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Date:   Thu Jun 27 13:48:18 2013 +0200

    drm/nouveau: make flipping lockdep safe

Acked-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: nouveau@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201127163528.2221671-1-daniel.vetter@ffwll.ch