nouveau: fix build eror when VGA_SWITCHEROO is not enabled

nouveau: fix build eror when VGA_SWITCHEROO is not enabled

Fix nouveau build error on x86, when ACPI is enabled but VGA_SWITCHEROO is
not enabled, by providing a stub function.

drivers/built-in.o: In function `nouveau_pmops_runtime_suspend':
nouveau_drm.c:(.text+0x3aac89): undefined reference to `nouveau_switcheroo_optimus_dsm'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>