Fix incorrect positioning of table view indicator arrows under Retina displays on Mac

The QStyleHelper::dpiScaled() function incorrectly used the
device pixel ratio to scale the input value.

This changes the function to always use a scale factor of
1.0 on Mac. This matches the current behavior of the
function in Qt 5 based on 3a685dcd5ef3b9e264abd7aacc9a8daffdb67992

MD-19976
test: Compile 'basic sorting' itemview example with Qt and run under
retina display, verify that table view indicator is drawn correctly.