drm/nouveau/dispnv50/headc57d: Make local function 'headc57d_olut' static

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/nouveau/dispnv50/headc57d.c:173:1: warning: no previous prototype for ‘headc57d_olut’ [-Wmissing-prototypes]

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/10