drm/amd/display: move FPU related code from dcn316 to dml/dcn31 folder

Moves FPU-related structs and dcn316_update_bw_bounding_box from dcn316
driver to dml/dcn31 that centralize FPU operations for DCN 3.1x

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>