drm/amd/display: Fix null check of pipe_ctx->plane_state for update_dchubp_dpp

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Fix null check of pipe_ctx->plane_state for update_dchubp_dpp

Similar to commit 6a057072ddd1 ("drm/amd/display: Fix null check for
pipe_ctx->plane_state in dcn20_program_pipe") that addresses a null
pointer dereference on dcn20_update_dchubp_dpp. This is the same
function hooked for update_dchubp_dpp in dcn401, with the same issue.
Fix possible null pointer deference on dcn401_program_pipe too.

(cherry picked from commit d8d47f739752227957d8efc0cb894761bfe1d879)