mxsfb display path: support DRM_IOCTL_MODE_SETGAMMA

Needed for e.g. nightlight to work.

AFAIK we can't do that in mxsfb itself. There's other places that come to mind but I'll label that mxsfb since this is for the mxsfb -> nwl -> phy -> panel pipeline.

(known issue but lets have a bug)