canGrow indicates to the DDX that the driver can enlarge the desktop via the
xf86_config->funcs->resize hook.  If so, xf86InitialConfiguration will set
virtual[XY] to match the configuration it chooses and will leave the crtc config
size ranges alone.  If FALSE, it will bloat the screen to fit the largest probed
mode and also set the crtc config max size to limit the desktop to the initial
virtual[XY] size.