MAINTAINERS: handle fbdev changes through drm-misc tree

fbdev patches will now go to upstream through drm-misc tree (IOW
starting with v5.4 merge window fbdev changes will be included in
DRM pull request) for improved maintainership and better integration
testing. Update MAINTAINERS file accordingly.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>