drm/i915: Store the active context object on all engines upon error

With execlists, we have context objects everywhere, not just RCS. So
store them for post-mortem debugging. This also has a secondary effect
of removing one more unsafe list iteration with using preserved state
from the hanging request. And now we can cross-reference the request's
context state with that loaded by the GPU.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-3-git-send-email-chris@chris-wilson.co.uk