drm/radeon/kms: fix IH writeback on r6xx+ on big endian machines

agd5f: fix commit message.

Signed-off-by: Cedric Cano <ccano@interfaceconcept.com>
Reviewed-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>