drm/i915: Clear the last_retired_context following a hang/reset

drm/i915: Clear the last_retired_context following a hang/reset

Following a hang and reset, we know that the engine is idle and all
context state has been saved or lost. Consequently, we know that the
engine is no longer referencing the last context and we can relinquish
our tracking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170212172002.23072-5-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>