agp/intel: Rename intel-gtt symbols

Exporting the symbols like intel_gtt_* creates some confusion inside
i915 that has symbols named similarly. In an attempt to isolate
platforms needing intel-gtt.ko, commit 7a5c9223

 ("drm/i915/gt: Split
intel-gtt functions by arch") moved way too much
inside gt/intel_gt_gmch.c, even the functions that don't callout to this
module. Rename the symbols to make the separation clear.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617230559.2109427-1-lucas.demarchi@intel.com