drm/radeon/kms: fix r6xx+ scanout on BE systems

R6xx+ have per-block swappers.  BE content in the
framebuffer will now be swapped properly during scanout.
Untested, however, the same code is reported working in
the UMS ddx.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>