Fallback on xrandr to get linux DPI info.

xdpyinfo is not always available on Linux (even with a DE), whereas
xrandr is (AFAICT) at least dependency of KDE and XFCE.  But xrandr
fails when used remotely, so keep it too.
...and again, you could check to see if `xrandr` exists, and if not, then throw an error