Update mount point after mount operations

Summary:
The order of udisks evaluation has changed from:

call Mount
propertiesChanged
mount call returns

call Mount
mount call returns
propertiesChanged

The mount has finished, but the property is not yet updated.

Solid caches properties, updating them when they change. This worked
before but due to the re-ordering client code gets "setupDone" requests
the mount point gets an outdated version from the cache and we get
errors. Invalidating the cache causes us to round-trip to the udisks
daemon meaning we'll have the correct values.

BUG: 370975

Test Plan:
Diagnosed but with dbus-monitor trace
Asked someone on the bug report to test this

Reviewers: Plasma

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12648