drm/i915: Purge i915_gem_object_is_dead()

i915_gem_object_is_dead() was a temporary lockdep aide whilst
transitioning to a new locking structure for obj->mm. Since commit
1233e2db ("drm/i915: Move object backing storage manipulation to its
own locking") it is now unused and should be removed.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308132629.7987-2-chris@chris-wilson.co.uk