drm/nv04-nv40: Fall back to panel rescaling if we have no usable native mode.

This allows the user to set a mode larger than the native one, useful
if we had trouble finding the actual native mode (e.g. because it goes
above the hardware bandwidth limits).

Reported-by: Grzesiek Sójka <pld@pfu.pl>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>