efi/libstub: Refactor and clean up GOP resolution picker code

The EFI stub implements various ways of setting the resolution of the
EFI framebuffer at boot, and this duplicates a lot of boilerplate for
iterating over the supported modes and extracting the resolution and
color depth.

Refactor this into a single helper that takes a callback, and use it for
the 'auto', 'list' and 'res' selection methods.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>