loader.efi: Do not use as frame buffer BLT-only GOPs.

Kernel needs physical frame buffer address and size, which Block
Transfer-only Graphics Output Protocol instances do not have.

Some recent ASUS boards like PRIME Z690M-PLUS D4 and PRIME H570-Plus
report two GOPs, out of which the second one support ConOut protocol,
that made it preferable, but is BLT-only, that made console unusable.

Discussed with: tsoome (previous version)
MFC after: 1 week