drm/i915/selftests: Fix mutex imbalance for igt_render_engine_reset_fallback

Smatch spots:

drivers/gpu/drm/i915/selftests/intel_hangcheck.c:669 igt_render_engine_reset_fallback() error: double unlock 'mutex:&i915->drm.struct_mutex'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170623131907.24236-1-chris@chris-wilson.co.uk


Reviewed-by: Michel Thierry <michel.thierry@intel.com>