drm/amd/display: move idle pipe allocation logic into dcn specific layer

[why]
generic dc resource file should not know what an optimal idle pipe is
because this is dcn hardware dependent.

[how]
We move the optimial pipe searching logic in dcn specific layer.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>