drm/mipi-dsi: Set the fwnode for mipi_dsi_device

After commit 3fb16866

 ("driver core: fw_devlink: Make cycle
detection more robust"), fw_devlink prints an error when consumer
devices don't have their fwnode set. This used to be ignored silently.

Set the fwnode mipi_dsi_device so fw_devlink can find them and properly
track their dependencies.

This fixes errors like this:
[    0.334054] nwl-dsi 30a00000.mipi-dsi: Failed to create device link with regulator-lcd-1v8
[    0.346964] nwl-dsi 30a00000.mipi-dsi: Failed to create device link with backlight-dsi

Reported-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Link: https://lore.kernel.org/lkml/2a8e407f4f18c9350f8629a2b5fa18673355b2ae.camel@puri.sm/
Fixes: 068a0023

 ("drm: Add MIPI DSI bus support")
Signed-off-by: Saravana Kannan <saravanak@google.com>
Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Link: https://lore.kernel.org/r/20230310063910.2474472-1-saravanak@google.com
Signed-off-by: Maxime Ripard <maxime@cerno.tech>