efi/libstub: Use relocated version of kernel's struct screen_info

In some cases, we expose the kernel's struct screen_info to the EFI stub
directly, so it gets populated before even entering the kernel.  This
means the early console is available as soon as the early param parsing
happens, which is nice. It also means we need two different ways to pass
this information, as this trick only works if the EFI stub is baked into
the core kernel image, which is not always the case.

Huacai reports that the preparatory refactoring that was needed to
implement this alternative method for zboot resulted in a non-functional
efifb earlycon for other cases as well, due to the reordering of the
kernel image relocation with the population of the screen_info struct,
and the latter now takes place after copying the image to its new
location, which means we copy the old, uninitialized state.

So let's ensure that the same-image version of alloc_screen_info()
produces the correct screen_info pointer, by taking the displacement of
the loaded image into account.

Reported-by: Huacai Chen <chenhuacai@loongson.cn>
Tested-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/linux-efi/20230310021749.921041-1-chenhuacai@loongson.cn/
Fixes: 42c8ea3d

 ("efi: libstub: Factor out EFI stub entrypoint into separate file")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>