OvmfPkg: QemuVideoDxe: simplify UEFI driver model use in Supported() / Start()
    
    A bus driver is allowed to ignore the actual value of RemainingDevicePath
    in Supported() and Start(), and to produce all child handles at once.
    
    This in effect means the following invariants for QemuVideoDxe:
    - (RemainingDevicePath == NULL), and
    - (Private->GopDevicePath != NULL)
    
    Simplify Supported() and Start() by substituting constant TRUE and FALSE
    (as appropriate) in expressions that check RemainingDevicePath and/or
    Private->GopDevicePath.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>