drm/panel: raspberrypi-touchscreen: Prevent double-free

The mipi_dsi_device allocated by mipi_dsi_device_register_full() is
already free'd on release.

Fixes: 2f733d61

 ("drm/panel: Add support for the Raspberry Pi 7" Touchscreen.")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720134525.563936-9-maxime@cerno.tech