drm/i915: unreference default context on module unload

Before module unload is called, gpu_idle() will switch
to default context. This will increment ref count of base
object as the default context is 'running' on module unload
time. Unreference the drm object so that when context
is freed, base object is freed as well.

v2: added comment to explain the refcounts (Ben Widawsky)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -274,6 +274,14 @@ void i915_gem_context_fini(struct drm_device *dev)
 	intel_gpu_reset(dev);
 
 	i915_gem_object_unpin(dctx->obj);
+
+	/* When default context is created and switched to, base object refcount
+	 * will be 2 (+1 from object creation and +1 from do_switch()).
+	 * i915_gem_context_fini() will be called after gpu_idle() has switched
+	 * to default context. So we need to unreference the base object once
+	 * to offset the do_switch part, so that i915_gem_context_unreference()
+	 * can then free the base object correctly. */
+	drm_gem_object_unreference(&dctx->obj->base);
 	i915_gem_context_unreference(dctx);
 }