drm/kms/radeon: pick digitial encoders smarter. (v3)

booting a Lenovo W500 with LVDS + DP outputs showed up a TODO we had
on our list, to pick a correct digital encoder block. The LVTMA
encoder requires the second digital encoder, all others can use any
encoder at all.

This fixes the digital encoder selection logic to enable LVDS/DP combos
to work okay.

V2: fix silly addition of connector dig_block and cleanup the other
places in the code that pick the encoder.

V3: rename to dig_encoder and clean up further - also fix
the picking algorithm.

tested on Lenovo W500 + desktop 3650 cards.

Signed-off-by: Dave Airlie <airlied@redhat.com>