xcb: Fix updating physical screen size

X server may return an empty physical screen size, for example
on VNC, Xephyr or some not very well supported hardware.
In this case it's possible to use the size of the virtual desktop,
but until now it was done only in the QXcbScreen constructor.

Move it to QXcbScreen::updateGeometry() and calculate physical screen
size using the DPI of the virtual desktop.

Task-number: QTBUG-45564
Change-Id: I6b757818a2fcefdd7b2c0aa31b840a88d625d6ae
Reviewed-by: Daniel Vrátil <dvratil@redhat.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>