drm/radeon/kms: fix DP detect and EDID fetch for DP bridges

Sink type is always DP for DP bridges and EDID fetch on
DP bridges is always i2c over aux rather than plain i2c.

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