drm/radeon/kms: fix tiling reg on fusion

The location of MC_ARB_RAMCFG changed on fusion.
I've diffed all the other regs in evergreend.h and this
is the only other reg that changed.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>