drm/amd/display: Add function to get the pipe from the stream context

[Why]
We need a helper function in dc to grab the pipe from the stream
context.

[How]
Add it.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>