drm/i915: Mark unmappable GGTT entries as PIN_HIGH

We allocate a few objects into the GGTT that we never need to access via
the mappable aperture (such as contexts, status pages). We can request
that these are bound high in the VM to increase the amount of mappable
aperture available. However, anything that may be frequently pinned
(such as logical contexts) we want to use the fast search & insert.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470832906-13972-1-git-send-email-chris@chris-wilson.co.uk