vulkan/wsi/x11: Increase the effective min. images for mailbox.

We need 5 images:
1) CPU work
2) GPU work
3) idle
4) queued for flip
5) presenting

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
13 jobs for !920 with wsi-mailbox-wait in 8 minutes and 26 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Containers Build
passed debian #449938

00:00:19

 
  Build+Test
passed meson-arm64 #449947

00:01:52

passed meson-armhf #449946

00:02:01

passed meson-clang #449940

00:07:35

passed meson-clover #449944

00:08:05

passed meson-i386 #449948

00:01:30

passed meson-main #449943

00:05:39

passed meson-swr-glvnd #449939

00:05:24

passed meson-vulkan #449945

00:02:05

passed scons-llvm #449950

00:02:45

passed scons-nollvm #449949

00:02:45

passed scons-swr #449941

00:04:39

passed scons-win64 #449942

00:06:21