gpu: ipu-v3: prg: avoid possible array underflow

gcc-8 reports that we access an array with a negative index
in an error case:

drivers/gpu/ipu-v3/ipu-prg.c: In function 'ipu_prg_channel_disable':
drivers/gpu/ipu-v3/ipu-prg.c:252:43: error: array subscript -22 is below array bounds of 'struct ipu_prg_channel[3]' [-Werror=array-bounds]

This moves the range check in front of the first time that
variable gets used.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/ipu-v3/ipu-prg.c

@@ -250,10 +250,14 @@ void ipu_prg_channel_disable(struct ipuv3_channel *ipu_chan)
 {
 	int prg_chan = ipu_prg_ipu_to_prg_chan(ipu_chan->num);
 	struct ipu_prg *prg = ipu_chan->ipu->prg_priv;
-	struct ipu_prg_channel *chan = &prg->chan[prg_chan];
+	struct ipu_prg_channel *chan;
 	u32 val;
 
-	if (!chan->enabled || prg_chan < 0)
+	if (prg_chan < 0)
+		return;
+
+	chan = &prg->chan[prg_chan];
+	if (!chan->enabled)
 		return;
 
 	pm_runtime_get_sync(prg->dev);
@@ -280,13 +284,15 @@ int ipu_prg_channel_configure(struct ipuv3_channel *ipu_chan,
 {
 	int prg_chan = ipu_prg_ipu_to_prg_chan(ipu_chan->num);
 	struct ipu_prg *prg = ipu_chan->ipu->prg_priv;
-	struct ipu_prg_channel *chan = &prg->chan[prg_chan];
+	struct ipu_prg_channel *chan;
 	u32 val;
 	int ret;
 
 	if (prg_chan < 0)
 		return prg_chan;
 
+	chan = &prg->chan[prg_chan];
+
 	if (chan->enabled) {
 		ipu_pre_update(prg->pres[chan->used_pre], *eba);
 		return 0;