mi: Add a default no-op miSourceValidate

Slightly simplifies the callers since they don't need to check for
non-NULL anymore.

I do extremely hate the workarounds here to suppress misprite taking the
cursor down though. Surely there's a better way.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>

render/mipict.c

@@ -294,10 +294,8 @@ SourceValidateOnePicture(PicturePtr pPicture)
 
     pScreen = pDrawable->pScreen;
 
-    if (pScreen->SourceValidate) {
-        pScreen->SourceValidate(pDrawable, 0, 0, pDrawable->width,
-                                pDrawable->height, pPicture->subWindowMode);
-    }
+    pScreen->SourceValidate(pDrawable, 0, 0, pDrawable->width,
+                            pDrawable->height, pPicture->subWindowMode);
 }
 
 void