drm/mipi-dsi: Avoid potential NULL pointer dereference

The mipi_dsi_packet_create() function dereferences the msg pointer
before checking that it's valid. Move the dereference down to where it
is required to avoid potentially dereferencing a NULL pointer.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/drm_mipi_dsi.c

@@ -323,8 +323,6 @@ EXPORT_SYMBOL(mipi_dsi_packet_format_is_long);
 int mipi_dsi_create_packet(struct mipi_dsi_packet *packet,
 			   const struct mipi_dsi_msg *msg)
 {
-	const u8 *tx = msg->tx_buf;
-
 	if (!packet || !msg)
 		return -EINVAL;
 
@@ -353,8 +351,10 @@ int mipi_dsi_create_packet(struct mipi_dsi_packet *packet,
 		packet->header[2] = (msg->tx_len >> 8) & 0xff;
 
 		packet->payload_length = msg->tx_len;
-		packet->payload = tx;
+		packet->payload = msg->tx_buf;
 	} else {
+		const u8 *tx = msg->tx_buf;
+
 		packet->header[1] = (msg->tx_len > 0) ? tx[0] : 0;
 		packet->header[2] = (msg->tx_len > 1) ? tx[1] : 0;
 	}