drm/amd/display: add get primary dpp pipe resource interface

[why]
Need to have a helper function to find the primary dp pipe of the plane
associated with a dpp pipe

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Wayne Lin <wayne.lin@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>