drm/radeon/kms: fix possible mis-detection of sideport on rs690/rs740

Check ulBootUpMemoryClock on AMD IGPs.

Fix regression noticed by Torsten Kaiser <just.for.lkml@googlemail.com>

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