drm/nouveau/kms: allow 225/297MHz pixel clocks for HDMI on Fermi/Kepler

Some Fermi's apparently alow allow 297MHz clocks, so create a parameter
which allows end-users to set it themselves until we have a reliable way
to determine the board's maximum pixel clocks.

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