drm/amd/display: Move bounding box to FPU folder

The final part of the DCN32 code that uses FPU is the bounding box code,
and this commit move it to dcn32_fpu.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>