Follow-up from "platforms/drm: drop the double tap timer"

The following discussion from !778 (merged) should be addressed:

• @davidedmundson started a discussion: (+1 comment)

  We also eventually want to close the libinput fd (would need separate patch though), to allow for touchscreen device to go into power saving state using runtime_pm.

  Yes. This won't make any difference until that's implemented.

  At which point I don't understand reason to remove this. If libinput is completely closed, then this code won't do anything as we won't process those double tap events in the first place.

  So removing it will only affect hardware that doesn't support hardware interrupts (like my lenovo convertible), so I don't really see the point.

  (I would support moving all this filter from the DRM backend code though)

• Move out drm input event filter out of the DRM backend code

• Close libinput fd when we are not using it (during DPMS off)