drm: drop unused 'magicfree' list

This list is write-only. It's never used for read-access, so no reason to
keep it around. Drop it!

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>