drm/radeon/kms: fix typo in IH_CNTL swap bitfield

commit fcb857ab

 upstream.

Only affects BE systems.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>