drm/i915: Enable eLLC caching of display buffers for SKL+

Since SKL the eLLC has been sitting on the far side of the system
agent, meaning the display engine can utilize it. Let's enable that.

I chose WB for the caching mode, because my numbers are indicating
that WT might actually be WB and WC might actually be UC. I'm not
100% sure that is indeed the case but at least my simple rendercopy
based benchmark didn't see any difference in performance.

Also if I configure things to do LLCeLLC+WT I still get cache dirt
on my screen, suggesting that is in fact operating in WB mode
anyway. This is also the reason I had to fix the MOCS target cache
to really say PTE rather than LLC+eLLC.
Since SKL the eLLC has been sitting on the far side of the system agent,
meaning the display engine can utilize it. Let's enable that.

Eero's earlier benchmarks numbers:
"* Results in GfxBench and Unigine (Valley/Heaven) tests were within daily
   variation on the tested SKL machines

 * SKL GT4e (128MB eLLC) / Wayland / Weston:
   +15-20% SynMark TexMem512 (512MB of textures)
   +4-6% SynMark TerrainFly*, CSCloth, ShMapVsm
   -5-10% SynMark TexMem128 (128MB of textures)

 * SKL GT3e (64MB eLLC) / Xorg / Unity:
   +4-8% GpuTest Triangle fullscreen (FullHD)
   -5-10% GpuTest Triangle windowed (1/2 screen)

 * SKL GT2 (no eLLC) / Xorg / Unity:
   * Some of the higher FPS SynMark pixel and vertex shader tests
     are few percent higher, more than daily variance
   => Do you see any reason why this machine would be impacted
      although it doesn't eLLC?"

Caveats:
- Still haven't tested with a prime setup
- Still not entirely sure this a good idea, but I've been
  using it on my cfl anyway :)

v2: Split the MOCS PTE change out

Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007120329.17076-3-ville.syrjala@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20201015122138.30161-3-chris@chris-wilson.co.uk