drm/amd/display: Handle potential dpp_inst mismatch with pipe_idx

[Why]
In some pipe harvesting configs, we will select the incorrect
dpp_inst when programming DTO. This is because when any intermediate
pipe is fused, resource instances are no longer in 1:1
correspondence with pipe index.

[How]
When looping through pipes to program DTO, get the dpp_inst
associated with each pipe from res_pool.

Signed-off-by: Anthony Wang <anthony1.wang@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>