drm/i915: Add module param to test the load detect code

This is useful for writing igts to make sure we don't break this,
without being forced to own a one of these dinosaurs.

Suggested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>