drm/nouveau/nv1a,nv1f/disp: fix memory clock rate retrieval

Based on the xf86-video-nv code, NFORCE (NV1A) and NFORCE2 (NV1F) have a
different way of retrieving clocks. See the
nv_hw.c:nForceUpdateArbitrationSettings function in the original code
for how these clocks were accessed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54587


Cc: stable@vger.kernel.org
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>