display: Don't always set the primary monitor to the first in list

As per the binding that we have between the list store and the combo-box, when
the first element is added to the list-store, the combo box set this value as
the selected-index, and this leads to a call to cc_display_monitor_set_primary
which set the first-listed monitor as primary and unset the real primary monitor.

To avoid this, just ignore the binding when rebuilding the UI, since in this
phase control-center should just reflect the actual state without changing
anything.

Fixes GNOME/gnome-control-center#419
11 jobs for primary-monitor-set-on-final-reload in 9 minutes and 7 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #248364
build

00:04:05

 
  Test
passed #248366
asan

00:05:00

passed #248365
test

00:04:23

 
  Manual
manual #248371
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manual #248367
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flatpak
manual #248370
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msan
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ppc64le
manual #248368
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tsan
manual #248369
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ubsan
 
  Review
manual #248373
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review
manual #248374
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