drm/nouveau: use drm_gem_plane_helper_prepare_fb

Instead of manually adjusting the plane state.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429134230.24334-3-christian.koenig@amd.com