NV20/NV30 is partially educated guesswork at this point, based on any
information around about available memory types and a horribly unspeakable
amount of vbios image scouring.  I'm not entirely certain the GDDR3 define
is correct, I have not spotted a single vbios with that value yet (though
it is mentioned in some 1218-using nv4x vbios), but there are reports that
some nv3x did use it..

NV40(100914) confirmed by switching an NV49 to DDR1/DDR2 values and making
sure that the binary driver behaviour showed it had detected DDR1/DDR2
instead of GDDR3 before dying horribly.

NV40(100474) confirmed by doing much the same task as above on an NV44,
except this was *much* easier as changing the values didn't seem to have
any noticable effect on the memory controller aside from changing the
binary driver's behaviour.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>