Default font is too small on Windows with Traditional Chinese UI language

Appears to be a regression due to https://codereview.qt-project.org/#/c/165708/

On Windows with a Traditional Chinese UI, the default UI font is PMingLiU (新細明體). On Qt 5.9.1 it uses the system-provided 9pt as the font size, which is nice and clear and matches the native UI of other (pre-Vista) applications (^). Qt 5.9.2 however decides to cap the system default font size to 8.25pt, which makes the text sub-optimal for reading. (Smaller fonts derived from the system default font size is even worse, because PMingLiU does not supply bitmap glyphs for ~7pt and the result rendering is all blurry.) 

This issue also only (and always) happens when there are more than one monitors (even when all are using the same DPI), because the above change only checks for the monitor count without checking the monitor DPI values.

 (This will likely be the same for Japanese and simplified Chinese.)

(^) Newer applications uses Microsoft JHengHei UI 10pt, which is the equivalent of Segoe UI 9pt for Traditional Chinese, and probably what QTBUG-58610 is about. (IIRC, but I might be wrong about the font size.)