uxa: Do not FreePixmap(screen->devPrivate)

uxa: Do not FreePixmap(screen->devPrivate)

Prevent the crash caused by

commit 444a1f7a8802999e27ecf5f6eb598df2206f7277
Author: Brendan King <brendan.king@imgtec.com>
Date:   Thu Apr 24 11:37:45 2014 +0100

    fb: fix screen pixmap leak on server reset

    Call FreePixmap() instead of free() to destroy the screen pixmap in
    fbCloseScreen().

    Signed-off-by: Frank Binns <frank.binns@imgtec.com>
    Reviewed-by: Keith Packard <keithp@keithp.com>
    Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>