drm/radeon/kms: fix some GART table entry bugs.

1. rv370 can accept 40-bit addresses - also at 24-bit shift not 4 bits
2. rs480 table can be in 40-bit space. - 4 bit shift for top 8 bits
3. rs480 table entries can be in 40-bit space.

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