drm/radeon/kms: rework radeon_dp_detect() logic

If the connector is eDP, it can only be DP, not TMDS.
Always set the detected sink type.  If the sink is
detected as non-DP, but there is no EDID, you can still
manually force the port on.  If the sink type is DP
and there's no DPCD, there's no way to force the monitor
on since you need both ends to train the link.

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