nouveau: fix nv40_perfctr_next() cleanup regression

gcc-6 warns about code in the nouveau driver that is obviously silly:

drivers/gpu/drm/nouveau/nvkm/engine/pm/nv40.c: In function 'nv40_perfctr_next':
drivers/gpu/drm/nouveau/nvkm/engine/pm/nv40.c:62:19: warning: self-comparison always evaluats to false [-Wtautological-compare]
  if (pm->sequence != pm->sequence) {

The behavior was accidentally introduced in a patch described as "This is
purely preparation for upcoming commits, there should be no code changes here.".
As far as I can tell, that was true for the rest of that patch except for
this one function, which has been changed to a NOP.

This patch restores the original behavior.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 8c1aeaa1 ("drm/nouveau/pm: cosmetic changes")
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>