drm/amd/display: Add symclk enable/disable during stream enable/disable

[Why & How]
Using dig_stream_mapper, program symclk_en and symclk_fe_src_sel when
enabling or disabling the corresponding stream.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Taimur Hassan <syed.hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>