drm/nouveau: use proper prototype in nouveau_pmops_runtime() definition

There is a prototype for this function in the header, but the function
itself lacks a 'void' in the argument list, causing a harmless warning
when building with 'make W=1':

drivers/gpu/drm/nouveau/nouveau_drm.c: In function 'nouveau_pmops_runtime':
drivers/gpu/drm/nouveau/nouveau_drm.c:730:1: error: old-style function definition [-Werror=old-style-definition]

Fixes: 321f5c5f ("drm/nouveau: replace multiple open-coded runpm support checks with function")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>