drm/i915/selftests: Drop unnecessary struct_mutex around i915_reset()

Since we no longer need to hold struct_mutex to perform a global device
reset, don't do so for igt_reset_wedge().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190215102732.15520-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/intel_hangcheck.c

@@ -399,10 +399,8 @@ static int igt_wedged_reset(void *arg)
 
 	i915_gem_set_wedged(i915);
 
-	mutex_lock(&i915->drm.struct_mutex);
 	GEM_BUG_ON(!i915_terminally_wedged(&i915->gpu_error));
 	i915_reset(i915, ALL_ENGINES, NULL);
-	mutex_unlock(&i915->drm.struct_mutex);
 
 	intel_runtime_pm_put(i915, wakeref);
 	igt_global_reset_unlock(i915);