drm/nouveau: dispnv50: fix missing-prototypes warning

nv50_display_create() is declared in another header, along with
a couple of declarations that are now outdated:

drivers/gpu/drm/nouveau/dispnv50/disp.c:2517:1: error: no previous prototype for 'nv50_display_create'

Fixes: ba801ef0 ("drm/nouveau/kms: display destroy/init/fini hooks can be static")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417210329.2469722-1-arnd@kernel.org