drm/i915: Always pin contexts into the high GGTT

Now that we have fast top-down insertion into the drm_mm, we can use it
for frequent runtime operations like insertion of the context object,
whereas before we limited it to the one-off insertion of the pinned
kernel context. Keeping the active context objects out of the mappable
region of the global GTT (except under memory pressure) improves our
ability to allocate mappable aperture region without triggering a GPU
stall.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170210101422.1598-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>