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

Similar to commit 6a057072 ("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.

Fixes: 63ab80d9 ("drm/amd/display: DML2.1 Post-Si Cleanup")
Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit d8d47f73)