drm/i915/gt: Re-do the intel-gtt split

Re-do what was attempted in commit 7a5c922377b4 ("drm/i915/gt: Split
intel-gtt functions by arch"). The goal of that commit was to split the
handlers for older hardware that depend on intel-gtt.ko so i915 can
be built for non-x86 archs, after some more patches. Other archs do not
need intel-gtt.ko.

Main issue with the previous approach: it moved all the hooks, including
the gen8, which is used by all platforms gen8 and newer. Re-do the
split moving only the handlers for gen < 6, which are the only ones
calling out to the separate module.

While at it do some minor cleanups:
- Rename the prefix s/gen5_/gmch_/ to be more accurate what platforms
are covered by intel_ggtt_gmch.c
- Remove dead code for gen12 out of needs_idle_maps()
- Remove TODO comment leftover
- Re-order if/else ladder in ggtt_probe_hw() to keep newest platforms
first

v2: Add minor cleanups (Matt Roper)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617230559.2109427-2-lucas.demarchi@intel.com