Don't unreference FBs of pixmaps from different screens in LeaveVT

FindClientResourcesByType finds pixmaps from all screens, but trying to
process ones from other screens here makes no sense and likely results
in a crash or memory corruption.

Fixes: c16ff42f ("Make all active CRTCs scan out an all-black
                      framebuffer in LeaveVT")
(Ported from radeon commit 2faaecc69b127248718e759c6c98c84d56dd1b6b)
12 jobs for pixmap_unref_fb-no-foreign-screens in 1 minute and 38 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-testing #665703

00:00:24

 
  Build
passed xserver-1.13 #665706

00:00:43

passed xserver-1.14 #665707

00:00:41

passed xserver-1.15 #665708

00:00:33

passed xserver-1.16 #665709

00:00:36

passed xserver-1.17 #665710

00:00:36

passed xserver-1.18 #665711

00:00:28

passed xserver-1.18-clang #665712

00:00:32

passed xserver-1.19 #665713

00:00:33

passed xserver-1.19-clang #665714

00:00:24

passed xserver-1.20 #665704

00:00:39

passed xserver-1.20-clang #665705

00:00:37