drm/panel/raspberrypi-touchscreen: Avoid NULL deref if not initialised

If a call to rpi_touchscreen_i2c_write from rpi_touchscreen_probe
fails before mipi_dsi_device_register_full is called, then
in trying to log the error message if uses ts->dsi->dev when
it is still NULL.

Use ts->i2c->dev instead, which is initialised earlier in probe.

Fixes: 2f733d6194 ("drm/panel: Add support for the Raspberry Pi 7" Touchscreen.")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220415162513.42190-2-stefan.wahren@i2se.com

drivers/gpu/drm/panel/panel-raspberrypi-touchscreen.c

@@ -229,7 +229,7 @@ static void rpi_touchscreen_i2c_write(struct rpi_touchscreen *ts,
 
 	ret = i2c_smbus_write_byte_data(ts->i2c, reg, val);
 	if (ret)
-		dev_err(&ts->dsi->dev, "I2C write failed: %d\n", ret);
+		dev_err(&ts->i2c->dev, "I2C write failed: %d\n", ret);
 }
 
 static int rpi_touchscreen_write(struct rpi_touchscreen *ts, u16 reg, u32 val)