drm/nouveau/fb/ramgt215: Restructure r111100 calculation for DDR2

Seems to be mostly equal to DDR3 on < GT218, should improve stability for
DDR2 reclocks.

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>