drm/radeon: fix r600 writeback setup.

This fixes 2 bugs:
1. the AGP calculation wasn't consistent with the PCI(E) calc for the
RPTR_ADDR registers. This consolidates the writes and fixes it up.

2. The scratch address was being incorrectly calculated, this breaks
it out into a lot more linear steps.

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