devm_drm_dev_alloc() can't allocate structures that embed a structure
which then again embeds drm_device. Workaround this by adding a
driver_private pointer to struct mipi_dbi_dev which the driver can use for
its additional state.

v3:
- Add documentation
Acked-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220227124713.39766-5-noralf@tronnes.org