Save the various pixmap functions before unwrapping

DestroyPixmap was restored by the driver but never saved before, guaranteeing that it was set to NULL during driver shutdown. This didn't matter because this code was never unwrapped properly due to a bug in the X server see xorg/xserver!1189.

Even if we did save it, it was already unconditionally overwritten before we got to uxa_driver_init(). So let's split that by moving the screen->*Pixmap changes to after uxa_driver_init() where we can now save them correctly, and restore them later in uxa_close_screen().

Fixes #17