drm/radeon: Fix the definition of RADEON_BUF_SWAP_32BIT

(Note that this is duplicated under various other names such
as R600_BUF_SWAP_32BIT etc...). At least now all the definitions
agree.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>