drm/v3d: Fix null pointer dereference of pointer perfmon

In the unlikely event that pointer perfmon is null the WARN_ON return path
occurs after the pointer has already been deferenced. Fix this by only
dereferencing perfmon after it has been null checked.

Fixes: 26a4dc29 ("drm/v3d: Expose performance counters to userspace")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424183512.1365683-1-colin.i.king@gmail.com

drivers/gpu/drm/v3d/v3d_perfmon.c

@@ -25,11 +25,12 @@ void v3d_perfmon_start(struct v3d_dev *v3d, struct v3d_perfmon *perfmon)
 {
 	unsigned int i;
 	u32 mask;
-	u8 ncounters = perfmon->ncounters;
+	u8 ncounters;
 
 	if (WARN_ON_ONCE(!perfmon || v3d->active_perfmon))
 		return;
 
+	ncounters = perfmon->ncounters;
 	mask = GENMASK(ncounters - 1, 0);
 
 	for (i = 0; i < ncounters; i++) {