drm/i915: Add debug module option for VTd validation

VTd has a few too many "outright disable the damn thing" workarounds
accumulated and for validation we want a simple knob to make sure we
disable them all.

Since this is for bdw+ validation and atm we don't have any
workarounds for bdw this option currently does nothing. So currently
this is just a placeholder to make sure reality will match with the
documented process for our validation people.

v2: Fix up param description (Jani).

v3: Actually git add ...

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>