drm/amd/display: DispalyPort: Write OUI only if panel supports it

[why]
Organizational Unit Identifier register is optional, and its
presence is published via Down Stream Port Count register.
Writing this register when not available will result in errors

[how]
Read this register and continue writing OUI only if the panel
has the support advertised.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>