mi: decouple miCreateScreenResources from pScreen->{width,height}

This allows pScreen->{width,height} to be changed to smaller values
after miScreenInit() is called but before pScreen is further set up
/ used by dix, so that the screen size can be fit to the mode of
choice even when it does not have the maximum width and/or height
among its peers.

XGetImage() for one will fail once the screen is change back to a
bigger size with xrandr, which prevents e.g. x11vnc to work on the
4 jobs for mi_screen_dev_private in 3 minutes and 43 seconds (queued for 7 seconds)