[Why]
dcn20_add_dsc_to_stream_resource is accessed in amdgpu_dm directly.
This creates build error for configuration with DCN disabled.

[How]
Make the function available through a resource pool function so
that dcn20 function need not be called directly.
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>