drm/i915/selftests: Trim struct_mutex duration for set-wedged selftest

Trim the struct_mutex hold and exclude the call to i915_gem_set_wedged()
as a reminder that it must be callable without struct_mutex held.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125132230.22221-4-chris@chris-wilson.co.uk