The mode_config->max_{width,height} is for the maximum size of a fb, not
the max scanout limits (of the layer-mixer).  It is legal, and in fact
common, to create a larger fb, only only scan-out a smaller part of it.
For example multi-monitor configurations for x11, or android wallpaper
layer (which is created larger than the screen resolution for fast
scrolling by just changing the src x/y coordinates).
Signed-off-by: Rob Clark <robdclark@gmail.com>