drm/nouveau: add nouveau_enum_find() util function

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/nouveau_util.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_util.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/nv50_graph.c
Simple merge