randr: Check rrPrivKey in RRHasScanoutPixmap()

RRHasScanoutPixmap() is called from xf86CheckHWCursor(), regardless of whether
or not RandR has been initialized.

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>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit 8eeff5d7)
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