drm/i915/gtt: Enable full-ppgtt by default everywhere

We should we have all the kinks worked out and full-ppgtt now works
reliably on gen7 (Ivybridge, Valleyview/Baytrail and Haswell). If we can
let userspace have full control over their own ppgtt, it makes softpinning
far more effective, in turn making GPU dispatch far more efficient by
virtue of better mm segregation.  On the other hand, switching over to a
different GTT for every client does incur noticeable overhead, but only
for very lightweight tasks.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180717095751.1034-1-chris@chris-wilson.co.uk