drm/nouveau/kms/nv04: remove set but unused variable.

The local variable `pclks` is defined and set but not used and can
therefore be removed.
Issue found by coccinelle.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>