drm/amd/display: Fixed extend to second screen mode hang

	1. Fixed acquire free split pipe bug.
	2. Change return value for dc_add_stream_to_ctx
	   from bool to enum.
	4. Remove redundant apply_ctx_for_surface calling
	5. Unlock pipe after back end programming.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>