drm/i915: Assert that the context-switch completion matches our context

When execlists signals the context completion, it also provides the
context id for the status event. Assert that id matches the one we expect.

v2: The upper dword of the context status is a duplicate of the upper
dword from elsp submission (i.e. includes the group id as well as the
context id). Include this check as well.
v3: Only check against lrc_desc (as this contains the hw_id check)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170123113132.18665-2-chris@chris-wilson.co.uk