drm/i915: Fix error checking/locking in perf/lookup_context()

1acfc104 missed to convert this one caller to be lockless. The side
effect of that was that the error check in lookup_context() became
incorrect. Convert now this caller too.

Fixes: 1acfc104 ("drm/i915: Enable rcu-only context lookups")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170714151242.517-1-imre.deak@intel.com