Move calls depending on $DISPLAY from startplasmacompositor to startplasma

Summary:
Xwayland is only available within startplasma, so only call xprop in
startplasma.
Additionally, the environment isn't complete within startplasmacompositor,
as $DISPLAY needs to be exported to the activation environment as well.

BUG: 377122

Test Plan: pinentry-qt works now when started from gpg-agent.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: graesslin, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9597