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)
Status Job ID Name Coverage
  Docker Image
passed #1004420
debian-testing

00:00:22

 
  Build
passed #1004423
xserver-1.13

00:00:21

passed #1004424
xserver-1.14

00:00:58

passed #1004425
xserver-1.15

00:00:47

passed #1004426
xserver-1.16

00:00:21

passed #1004435
xserver-1.17

00:00:37

passed #1004428
xserver-1.18

00:00:33

passed #1004429
xserver-1.18-clang

00:00:32

passed #1004430
xserver-1.19

00:00:29

passed #1004431
xserver-1.19-clang

00:00:37

passed #1004421
xserver-1.20

00:00:42

passed #1004422
xserver-1.20-clang

00:01:08

failed #1004427
xserver-1.17

00:02:18