[platforms/drm] Only show cursor if a pointer device is available

Summary:
When the pointer device gets plugged off the cursor is hidden, when
a pointer device gets plugged in the cursor is shown again.

This allows to easily turn a tablet into a proper system by showing
the cursor only when it makes sense.

Test Plan: Tested on exopc with external mouse

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1682