drm/i915: Remove obj->pin_mappable

The obj->pin_mappable flag only exists for debug purposes and is a
hindrance that is mistreated with rotated GGTT views. For debug
purposes, it suffices to mark objects with pin_display as being of note.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>