XCB: Don't recalculate the DPI if we have a forced DPI setting

logicalDpi() already has the logic to check m_forcedDpi, so let's reuse
it. I hope it's not a problem that we send a signal that it changed when
nothing changed.

[ChangeLog][Platform Specific Changes][X11 / XCB]Fixed a bug that caused
Qt applications to think the screen DPI had changed when it had not,
after connecting or disconnecting monitors.

Task-number: QTBUG-32683
Change-Id: I45dd27de5109e65e7599915f11cfdb633a65a67c
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>