drm/vc4: perfmon: Fix variable dereferenced before check

Commit 30f8c74c ("drm/vc4: Warn if some v3d code is run on BCM2711")
introduced a check in vc4_perfmon_get() that dereferences a pointer before
we checked whether that pointer is valid or not.

Let's rework that function a bit to do things in the proper order.
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 30f8c74c ("drm/vc4: Warn if some v3d code is run on BCM2711")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622080243.22119-1-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_perfmon.c

@@ -17,12 +17,15 @@
 
 void vc4_perfmon_get(struct vc4_perfmon *perfmon)
 {
-	struct vc4_dev *vc4 = perfmon->dev;
+	struct vc4_dev *vc4;
 
+	if (!perfmon)
+		return;
+
+	vc4 = perfmon->dev;
 	if (WARN_ON_ONCE(vc4->is_vc5))
 		return;
 
-	if (perfmon)
 	refcount_inc(&perfmon->refcnt);
 }