agp: Export intel_gtt_read_pte, required by i915kms

This isn't used by modern cards, but is needed for i915kms to load
on a system that has agp as a module not compiled into the kernel.

PR: 291214
Reviewed by: cy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53906

(cherry picked from commit a87c1e2dd8fc997d6ee603c252c543afe8a4d2a4)
(cherry picked from commit 8befd2d2b6a0c5d54fa53246a89ae09009007799)
(cherry picked from commit ae1f6fd47c4584d1abe5ecf26f291d282e6f6a67)