kcms/tablet: Always enable the "Map to screen" combo box

Previously this was disabled in single monitor setups, the assumption being that it's only interesting or worthwhile to set if you have more than one screen. Whilst this is true, there are two scenarios where we should consider enabling this:

First, in a future MR I will soon reduce the mapping options for "Follow the current screen" and single-monitor users must explicitly choose their monitor to access said options.

Second, there may be cases where a user has multiple screens but some may be off - appearing like a multimonitor setup. They should still be able to set a screen explicitly here.

Functionally, this doesn't change anything at the moment apart from re-enabling this combo box.