kms: Handle changes to SourceValidate call chain in xserver 19

xserver 19 expects the SourceValidate hook to always be filled in with
something valid. For earlier servers it's harmless to simply fill this
in with a do-nothing function instead of NULL.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
13 jobs for handle-source-validate-changes in 3 minutes and 43 seconds (queued for 52 seconds)