drm/amd/display: Call dsc related functions indirectly via dc interface

[Why&How]
Accessing dcn20_add_dsc_to_stream_resource directly
causes build failure for configuration which has
CONFIG_DRM_AMD_DC_DCN disabled. Fix this by
calling the corresponding function exposed via dc
resource functions.

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>