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>