display-panel: Show a scale row when using mirrored setup

The listed scaling values should be supported by all configured monitors in
order to have this working.

Fixes #297
11 jobs for only-show-appliable-global-scales in 6 minutes and 3 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #158223
build

00:03:38

 
  Test
passed #158225
asan

00:02:23

passed #158224
test

00:00:55

 
  Manual
manual #158230
allowed to fail manual
armv8
manual #158226
allowed to fail manual
flatpak
manual #158229
allowed to fail manual
msan
manual #158231
allowed to fail manual
ppc64le
manual #158227
allowed to fail manual
tsan
manual #158228
allowed to fail manual
ubsan
 
  Review
manual #158232
allowed to fail manual
review
manual #158233
allowed to fail manual
stop_review