QNX: Replaced physical screen size warning

On stock QNX it is quite common that libscreen doesn't provide the physical
screen size. The warning about this has been replaced with a debug statement,
because it was annoying especially when running auto tests.

Change-Id: Iab07f0d4a6293c40678f2ec3e77352e68797c8c7
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>