drm/i915: drop unused i915.disable_vtd_wa module parameter

This is a manual revert of

commit 7a10dfa6
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Apr 1 09:33:47 2014 +0200

    drm/i915: Add debug module option for VTd validation

as no users have appeared.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455621493-6865-2-git-send-email-jani.nikula@intel.com