loader.efi: only use firmware provided Blt on GOP

gfx_state.tg_private points to a EFI_GRAPHICS_OUTPUT_PROTOCOL only when
using GOP.

The firmware provided Blt functions on UGA platforms have been observed
to not work on old MacBooks, and are likley hit or miss anyways as UGA
has been deprecated since 2006.

Reviewed by: tsoome
PR: 291935
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54432