drm/i915: Track i915_active using debugobjects

Provide runtime asserts and tracking of i915_active via debugobjects.
For example, this should allow us to check that the i915_active is only
active when we expect it to be and is never freed too early.

One consequence is that, for simplicity, we no longer allow i915_active
to be on-stack which only affected the selftests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621183801.23252-2-chris@chris-wilson.co.uk