drm/nouveau/bios: add proper support for opcode 0x59

More analysis shows that this is identical to 0x79 except that it loads
the frequency indirectly from elsewhere in the VBIOS.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91025


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