drm/radeon/kms: fix sideport detection on newer rs880 boards

The meaning of ucMemoryType changed on recent boards, however,
ulBootUpSidePortClock should be set properly across all boards.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>