Update seat's timestamp after waking up screen through double tap

Summary:
The drm platform has a special handling for touch events while the
screen is in dpms. All touch events are filtered out, but a double
tap wakes up the screen.

Due to the filtering out of the events the idle timers were not reset.
So if you only double tapped, the screen turned on, but not off again.

This change updates the timestamp after double tap, so that the idle
timers are restarted. All other events are still filtered out, to not
have "fake" events (e.g. smart phone in pocket) wake up the device.

BUG: 392754
FIXED-IN: 5.12.6

Test Plan: only compile tested

Reviewers: #kwin, #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12668