drm/tinydrm: Fix memory leak in hx8357d_probe

[Markus Elfring]

> …
>> +free_dbidev:
>> +	kfree(dbidev);
> …
>
> I became curious if there is a need for such a memory release at another place.

I have taken another look also at a related implementation detail.
Now I have realised that the desired clean-up should usually be achieved by
the callback function “mipi_dbi_release” for this software module.
https://elixir.bootlin.com/linux/v5.4-rc2/source/drivers/gpu/drm/drm_mipi_dbi.c#L581
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_mipi_dbi.c?id=0365fb6baeb1ebefbbdad9e3f48bab9b3ccb8df3#n581

Regards,
Markus