drm/nouveau: nouveau: use correct string length

gcc-8 reports

drivers/gpu/drm/nouveau/nvkm/engine/pm/base.c: In function 'nvkm_perfmon_mthd':
include/linux/string.h:265:9: error: '__builtin_strncpy' specified bound 64 equals destination size [-Werror=stringop-truncation]

We need one less byte or call strlcpy() to make it a
nul-terminated string.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>