[WHY]
Vendor dongle requires propietary OUI and handshake sequence.

[HOW]
Add a new structure to dc_init_data, to allow creator to pass
an override vendor_oui. This value will be written to DP_SOURCE_OUI
instead of AMD signature, when dpcd_set_source_specific_data is
called.
Signed-off-by: Calvin Hou <Calvin.Hou@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>