drm/i915: release struct_mutex on the i915_gem_init_hw fail path

Release struct_mutex if init_rings() fails.

This is a regression introduced in
commit 35a57ffb
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Nov 20 00:33:07 2014 +0100

    drm/i915: Only init engines once

Reported-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>