DRM driver cleanups

Created by: JoseExposito

Hi!

I recently bought a StarFive VisionFive board and, since I'm trying to learn more about the DRM subsystem, I'm looking into the KMS driver code.

I don't know what's the status of the driver, if there's a list of pending features you'd like to implement or if you'd be interesting in sending the code to the staging tree and work upstream... So, this PR is my way of saying hello and get more information about the project, as well as a way to contribute some trivial patches.

Looking forward to your response, Jose