vga: raise xres+yres limits

The vgabios will check whenever any given video mode will fit into the
given video memory before adding it to the list of available modes, so
there is no need to keep xmax * ymax * 32bpp lower than VGA_RAM_SIZE.

Lets raise the limits a bit.  Should be good for a few years, display
sizes are not growing that fast.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
