dix: remove caching of drawables and graphics contexts. The security checks simply bypass the cached values so they are unused.

 Backported from X.org:

 commit 9a183d7ba50e31afa133cc03aee7991517a283ea
 Author: Eamon Walsh <ewalsh@tycho.nsa.gov>
 Date:   Tue Aug 14 11:39:26 2007 -0400

    dix: remove caching of drawables and graphics contexts.  The security checks
    simply bypass the cached values so they are unused.

 Backport to nx-libs: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>