drm/radeon/kms: vram sizing on certain r100 chips needs workaround.

If an rn50/r100/m6/m7 GPU has < 64MB RAM, i.e. 8/16/32, the
aperture used to calculate the MC_FB_LOCATION needs to be worked
out from the CONFIG_APER_SIZE register, and not the actual vram size.

TTM VRAM size was also being initialised wrong, use actual vram size
to initialise it.

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