drm/i915/guc: Unwind GuC workqueue reservation if request construction fails

We reserve space in the GuC workqueue for submitting the request in the
future. However, if we fail to construct the request, we need to give
that reserved space back to the system.

Fixes: dadd481b

 ("drm/i915/guc: Prepare for nonblocking execbuf submission")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97978
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161007065327.24515-4-chris@chris-wilson.co.uk