drm/i915: Make workaround verification *optional*

Sometimes the HW doesn't even play fair, and completely forgets about
register writes. Skip verifying known troublemakers.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108954
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190417075657.19456-4-chris@chris-wilson.co.uk