yuzu/loading_screen: Resolve runtime Qt string formatting warnings

In our error console, when loading a game, the strings:

QString::arg: Argument missing: "Loading...", 0
QString::arg: Argument missing: "Launching...", 0

would occasionally pop up when the loading screen was running. This was
due to the strings being assumed to have formatting indicators in them,
however only two out of the four strings actually have them.

This only applies the arguments to the strings that have formatting
specifiers provided, which avoids these warnings from occurring.