drm/i915/gt: Rename i915_gem_restore_ggtt_mappings() for its new placement

The i915_ggtt now sits beneath gt/ outside of the auspices of gem/ and
should be given a fresh name to reflect that. We also want to give it a
name that reflects its role in the system suspend/resume, with the
intention of pulling together all the GGTT operations (e.g. restoring
the fence registers once they are pulled under gt/intel_ggtt_detiler.c)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Rreviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200130181710.2030251-2-chris@chris-wilson.co.uk