drm/amd/display: move FPU related code from dcn315 to dml/dcn31 folder

Moves related structs and dcn315_update_bw_bounding_box from dcn315
driver code to dml/dcn31_fpu that centralizes FPU code 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>