drm/qxl: don't take vga ports on rev5+

qemu 5.0 introduces a new qxl hardware revision 5.  Unlike revision 4
(and below) the device doesn't switch back into vga compatibility mode
when someone touches the vga ports.  So we don't have to reserve the
vga ports any more to avoid that happening.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200807105501.24599-1-kraxel@redhat.com