radeon: Fix KMS CP writeback on big endian machines.

This is necessary even with PCI(e) GART, and it makes writeback work even with
AGP on my PowerBook. Might still be unreliable with older revisions of UniNorth
and other AGP bridges though.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Reviewed-by: Alex Deucher <alex.deucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>