drm/amdgpu: only set DP subconnector type on DP and eDP connectors

Fixes a crash in drm_object_property_set_value() because the property
is not set for internal DP ports that connect to a bridge chips
(e.g., DP to VGA or DP to LVDS).

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=210739
Fixes: 65bf2cf9

 ("drm/amdgpu: utilize subconnector property for DP through atombios")
Tested-By: Kris Karas <bugs-a17@moonlit-rail.com>
Cc: Oleg Vasilev <oleg.vasilev@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.10.x