RRProviderAutoConfigGpuScreen() is called from outside RandR, so there is no
guarantee that RandR has been initialized when it is called.

As mentioned in commit 4226c6d0, it's possible that RandR has not been
initialized if the server is configured with Xinerama and there is more than one
X screen. Calling rrGetScrPriv when RandR isn't initialized causes an assertion
failure that aborts the server:

  Xorg: ../include/privates.h:121: dixGetPrivateAddr: Assertion
  key->initialized' failed.

Just as in commit 4226c6d0

, fix the problem by checking
dixPrivateKeyRegistered(rrPrivKey) before calling rrGetScrPriv.

Signed-off-by: Alex Goins <agoins@nvidia.com>