dri2: Make createImageFromName() take a __DRIscreen instead of __DRIcontext

We can't expect to have a context when this is called, and we don't need one
so just require a __DRIscreen instead.

Reported by Yu Dai <yu.dai@intel.com>

include/GL/internal/dri_interface.h

@@ -805,7 +805,7 @@ typedef struct __DRIimageExtensionRec __DRIimageExtension;
 struct __DRIimageExtensionRec {
     __DRIextension base;
 
-    __DRIimage *(*createImageFromName)(__DRIcontext *context,
+    __DRIimage *(*createImageFromName)(__DRIscreen *screen,
 				       int width, int height, int format,
 				       int name, int pitch,
 				       void *loaderPrivate);