drm/amd/display: add dcn30_link_encoder_validate_output_with_stream to header

[Why]
dcn30_link_encoder_validate_output_with_stream was a static function.

[How]
remove the static define and include it in the header.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>