drm/radeon/kms: respect TOM on rs100->rs480 IGP variants.

Normally we are free to place VRAM where we want in the GPUs
memory address space, however on IGP chips the VRAM is actual RAM,
and no special translation or aperture is used inside the GPU MC.

So when you move the VRAM aperture away from the TOM register,
you actually move it into main memory and can trash things quite badly.

This commit makes the code respect the TOM location for MC_FB_LOCATION.

Signed-off-by: Dave Airlie <airlied@redhat.com>