OvmfPkg/VideoDxe: document EFI_EDID_OVERRIDE_PROTOCOL.GetEdid() call
    
    According to the UEFI spec -- and to the edk2 header
    "MdePkg/Include/Protocol/EdidOverride.h" too --,
    EFI_EDID_OVERRIDE_PROTOCOL_GET_EDID takes an (EFI_HANDLE*), and not an
    EFI_HANDLE, as second parameter ("ChildHandle").
    
    This is probably [*] a bug in the UEFI spec. Given that this CSM module
    (VideoDxe) had been used for a long time on physical platforms before it
    was moved to OvmfPkg, keep the current "ChildHandle" argument, just cast
    it explicitly.
    
    [*] The edk2 tree contains no other GetEdid() call, and also no GetEdid()
        implementation.
    
        The edk2-platforms tree contains two GetEdid() calls, at commit
        022c212167e0, in files
        - "Drivers/DisplayLink/DisplayLinkPkg/DisplayLinkGop/Edid.c",
        - "Drivers/OptionRomPkg/CirrusLogic5430Dxe/Edid.c".
    
        From these, the first passes an (EFI_HANDLE*) as "ChildHandle", the
        second passes an EFI_HANDLE. It's difficult to draw a conclusion. :/
    
    No functional changes.
    
    (I've also requested a non-normative (informative) clarification for the
    UEFI spec: <https://mantis.uefi.org/mantis/view.php?id=2018>, in the
    direction that matches this patch.)
    
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: David Woodhouse <dwmw2@infradead.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>