drm/nouveau/pm: restructure bios table parsing

It turns out we need access to some additional information in various VBIOS
tables to handle PFB memory timings correctly.

Rather than hack in parsing of the new stuff in some kludgy way, I've
restructured the VBIOS parsing to be more primitive, so we can use them in
more flexible ways in the future.

The perflvl->timing association code is disabled for the moment until it can
be reworked.  We don't use this stuff yet anyway, so no harm done.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Martin Peres <martin.peres@labri.fr>