App: use correct GPU name

[lenemter]

Fixes #581

The switcheroo_control.get_gpu_name (bool default_gpu) method, as its signature indicates, takes whether to return default GPU name. In case where app prefers non-default gpu, we need to get default name, and vice versa. But this was inversed when the switcheroo was introduced in #228

[flodavid]

Are you sure about this ? Since the parameter in Switcheroo is named default_gpu, it makes sense to negate the value of prefers_nondefault_gpu when we call it. If this change is necessary, it means there is something wrong in the Switcheroo method.
Edit: I got it now, sorry, we do want the opposite of the GPU preference here. The parameter name default_gpu in Switcheroo is a bit confusing though.