Reorder argument parsing in qlinuxfbscreen.cpp effects mmsize und size

The regex size=(\\d+)x(
d+) fits also to mmsize=(\\d+)x(
d+) , this is the
reason why the mmsize parameter is parsed as size.

You can reproduce this e.g. with following parameters:
"-platform linuxfb:fb=/dev/fb1:mmsize=800x480:size=800x480:offset=50x50"

This patch changes the order of argument parsing, so that the mmsize parameter
comes first.