display-factory: Correctly return from idle callback

The idle callback was defined as a void function rather than returning a
boolean as it should. This meant that the return value may end up being
non-zero in some situations causing an infinite loop.

This seems to only happen on aarch64 and possibly only with certain
compiler versions.

Closes: #668
1 job for benzea/fix-idle-cb-return in 3 minutes and 59 seconds
Status Name Job ID Coverage
passed build-fedora #1224718