drm/nv50: add memory type detection

DDR1/DDR[23] confirmed on NVA8 (see note about DDR3 in source) by changing
the value and watching the binary driver's behaviour.

GDDR3/4 values confirmed on a NV96 via the same method above.  That GDDR4
is present is interesting, as far as I can see no boards using it were ever
released.

GDDR5 value is based on VBIOS images of known GDDR5 boards.

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