drm/i915: Nuke intel_modeset_preclose

Now that the drm core unlinks/disarms events there's no need to do so
ourselves anymore. Nuke the code.

Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-4-git-send-email-daniel.vetter@ffwll.ch