loader.efi: probe for UGA if GOP isn't found

Probe for UGA instead of returning early if we can't find GOP.

Reviewed by: tsoome
PR: 291935
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54431

(cherry picked from commit 5d85dde27b4769604fc108b89328607e70e767ed)