drm/radeon/kms: fix radeon_dp_get_modes for LVDS bridges (v2)

Need to call ExternalEncoderControl to set up DDC before
trying to get an EDID for all DP bridge chips (including
DP to LVDS).

Also remove redundant encoder assignment.

V2: fix typo in commit message.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>