drm: mipi-dsi: make mipi_dsi_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the mipi_dsi_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20240203-bus_cleanup-gpu-v1-2-1b6ecdb5f941@marliere.net


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240203-bus_cleanup-gpu-v1-2-1b6ecdb5f941@marliere.net