drm/i915/dp: downstream port capabilities are not present in DPCD 1.0

We haven't read the downstream port caps for DPCD 1.0, so don't use
them.

v2: use defines for DPCD 1.0 downstream port types

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>