tone_mapping: add missing unit conversion in pl_tone_map_params_infer()

output_max and input_max might not use the same scaling function, so it's not safe to compare them directly.

Note: I'm not sure if this affects any current uses of the function. I only noticed it while hacking up the code locally for the purpose of comparing mpv's tone mapping to a separate implementation I'm working on. Still, it's good to fix regardless.