drm/i915: Rename i915_gem_engine_cleanup() to engine_set_wedged()

It has been some time since i915_gem_engine_cleanup was only called from
the module unload path, and now it is only called when the GPU is
wedged. Mika complained that the name is confusing, especially in light
of the existence of i915_gem_cleanup_engines().

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>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170110172246.27297-5-chris@chris-wilson.co.uk