drm/radeon/kms: fix tv dac conflict resolver

On systems with the tv dac shared between DVI and TV,
we can only use the dac for one of the connectors.
However, when using a digital monitor on the DVI port,
you can use the dac for the TV connector just fine.
Check the use_digital status when resolving the conflict.

Fixes fdo bug 27649, possibly others.

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