platforms/drm: fix under- and overscan

First commit: the enums belong to "underscan", not to "underscan hborder", and the values for aspect ratio need to be switched. Should be backported when 925ff964 is

Second commit: under/overscan wasn't applied until modeset, making it pretty useless. Should be backported now

amdgpu seems to have underscan properties available for all outputs, not just broken TVs. I could test it now and it works like a charm