drm/radeon/kms: adjust eDP handling (v2)

eDP is usually used as an LVDS replacement, so treat
it more like LVDS from the user perspective.

v2: encoder mode is always DP for eDP.

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