drm/i915: Rename obj->pin_display to obj->pin_global

In the next patch, we want to extend use of the global pin counter for
semi-permanent pinning of context/ring objects. Given that we plan to
extend the usage to encompass a disparate set of objects, we want a name
that reflects both and should entail less confusion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013202621.7276-2-chris@chris-wilson.co.uk