drm/i915/selftests: Release ctx->engine_mutex after iteration

A lock once taken must be released again.

Fixes: c31c9e82

 ("drm/i915/selftests: Teach switch_to_context() to use the context")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022223316.12662-1-chris@chris-wilson.co.uk