drm/nouveau/fb/gf100-: rework ram detection

This commit reworks the RAM detection algorithm, using RAM-per-LTC to
determine whether a board has a mixed-memory configuration instead of
using RAM-per-FBPA.  I'm not certain the algorithm is perfect, but it
should handle all currently known configurations in the very least.

This should fix GTX 970 boards with 4GiB of RAM where the last 512MiB
isn't fully accessible, as well as only detecting half the VRAM on
GF108 boards.

As a nice side-effect, GP10x memory detection now reuses the majority
of the code from earlier chipsets.

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