drm/i915: vfuncs for gtt_clear_range/insert_entries

We have a few too many differences here, so finally take the prepared
abstraction and run with it. A few smaller changes are required to get
things into shape:

- move i915_cache_level up since we need it in the gt funcs
- split up i915_ggtt_clear_range and move the two functions down to
  where the relevant insert_entries functions are
- adjustments to a few function parameter lists

Now we have 2 functions which deal with the gen6+ global gtt
(gen6_ggtt_ prefix) and 2 functions which deal with the legacy gtt
code in the intel-gtt.c fake agp driver (i915_ggtt_ prefix).

Init is still a bit a mess, but honestly I don't care about that.

One thing I've thought about while deciding on the exact interfaces is
a flag parameter for ->clear_range: We could use that to decide
between writing invalid pte entries or scratch pte entries. In case we
ever get around to fixing all our bugs which currently prevent us from
filling the gtt with empty ptes for the truly unused ranges ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwidawsk: Moved functions to the gtt struct]
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>