Move from the xf86 specific ScrnInfoRec::privates, to the dix private
handling. Since there's no FreeScreen function in ScreenPtr, fold the
former within the existing CloseScreen.
Users, such as modesetting are updated, and out of tree drivers will
need equivalent, yet trivial, patch.
Note: we need to ensure that the screen private is unset and the screen
callbacks are restored in our CloseScreen function.
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>