drm/i915: Add a wakeref getter for iff the wakeref is already active

For use in the next patch, we want to acquire a wakeref without having
to wake the device up -- i.e. only acquire the engine wakeref if the
engine is already active.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626154549.10066-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_wakeref.h

@@ -71,6 +71,21 @@ intel_wakeref_get(struct intel_runtime_pm *rpm,
 	return 0;
 }
 
+/**
+ * intel_wakeref_get_if_in_use: Acquire the wakeref
+ * @wf: the wakeref
+ *
+ * Acquire a hold on the wakeref, but only if the wakeref is already
+ * active.
+ *
+ * Returns: true if the wakeref was acquired, false otherwise.
+ */
+static inline bool
+intel_wakeref_get_if_active(struct intel_wakeref *wf)
+{
+	return atomic_inc_not_zero(&wf->count);
+}
+
 /**
  * intel_wakeref_put: Release the wakeref
  * @i915: the drm_i915_private device