OvmfPkg: QemuVideoDxe: clarify QEMU_VIDEO_MODE_DATA.ModeNumber
    
    The field name "ModeNumber" in QEMU_VIDEO_MODE_DATA is misleading -- it is
    not immediately obvious whether this field carries a client-visible mode
    number, in the GOP sense, or an internal, card type specific mode index.
    After checking all references, rename the field to "InternalModeIndex".
    
    Also, when filling in the card type independent QEMU_VIDEO_MODE_DATA array
    from the card type specific mode array, distinguish the GOP mode number
    from the internal mode index in the debug message.
    
    This patch effects no functional changes.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>