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.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>