drm/radeon/kms: fix VRAM sizing like DDX does it.

Doing this like the DDX seems like the most sure fire way to avoid
having to reinvent it slowly and painfully. At the moment we keep
getting things wrong with aper vs vram, so we know the DDX does it right.

booted on PCI r100, PCIE rv370, IGP rs400.

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