drm/nv50: hopefully handle the DDR2/DDR3 memtype detection somewhat better

M version 2 appears to have a table with some form of memory type info
available.

NVIDIA appear to ignore the table information except for this DDR2/DDR3
case (which has the same value in 0x100714).  My guess is this is due to
some of the supported memory types not being represented in the table.

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