dri: don't set the dixPrivate key to NULL, as this is a staticly set variable.

- This breaks on the 2nd server generation.
- No other subsystem seems to NULL their key.
- This should fix bug 17982.

hw/xfree86/dri/dri.c

@@ -351,7 +351,6 @@ DRIScreenInit(ScreenPtr pScreen, DRIInfoPtr pDRIInfo, int *pDRMFD)
     pDRIPriv = (DRIScreenPrivPtr) xcalloc(1, sizeof(DRIScreenPrivRec));
     if (!pDRIPriv) {
 	dixSetPrivate(&pScreen->devPrivates, DRIScreenPrivKey, NULL);
-        DRIScreenPrivKey = NULL;
         return FALSE;
     }
 
@@ -744,7 +743,6 @@ DRICloseScreen(ScreenPtr pScreen)
 
 	xfree(pDRIPriv);
 	dixSetPrivate(&pScreen->devPrivates, DRIScreenPrivKey, NULL);
-	DRIScreenPrivKey = NULL;
     }
 }