drm/i915: Only activate i915_active debugobject once

The point of debug_object_activate is to mark the first, and only the
first, acquisition. The object then remains active until the last
release.  However, we marked up all successful first acquires even though
we allowed concurrent parties to try and acquire the i915_active
simultaneously (serialised by the i915_active.mutex).

Testcase: igt/gem_mmap_gtt/fault-concurrent
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827132631.18627-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_active.c

@@ -92,11 +92,15 @@ static void debug_active_init(struct i915_active *ref)
 
 static void debug_active_activate(struct i915_active *ref)
 {
+	lockdep_assert_held(&ref->mutex);
+	if (!atomic_read(&ref->count)) /* before the first inc */
 		debug_object_activate(ref, &active_debug_desc);
 }
 
 static void debug_active_deactivate(struct i915_active *ref)
 {
+	lockdep_assert_held(&ref->mutex);
+	if (!atomic_read(&ref->count)) /* after the last dec */
 		debug_object_deactivate(ref, &active_debug_desc);
 }