drm/i915: Drop mutex after successful kref_put_mutex()

The kref_put_mutex() returns with the mutex held after freeing the
object - so we must remember to drop it...

Fixes: 69df05e11ab8 ("drm/i915: Simplify releasing context reference")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161219101357.28140-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -3520,9 +3520,10 @@ static inline void i915_gem_context_put(struct i915_gem_context *ctx)
 
 static inline void i915_gem_context_put_unlocked(struct i915_gem_context *ctx)
 {
-	kref_put_mutex(&ctx->ref,
-		       i915_gem_context_free,
-		       &ctx->i915->drm.struct_mutex);
+	struct mutex *lock = &ctx->i915->drm.struct_mutex;
+
+	if (kref_put_mutex(&ctx->ref, i915_gem_context_free, lock))
+		mutex_unlock(lock);
 }
 
 static inline struct intel_timeline *