drm/i915/selftests: Ignore coherency failures on Broadwater

We've been ignoring similar coherency issues in IGT for Broadwater, and
specifically Broadwater (original gen4) and not, for example, Crestline
(same generation as Broadwater, but the mobile variant). Without any
means to reproduce locally (I have a 965GM but alas no 965G), fixing will
be slow, so tell CI to ignore any failure until we are ready with a fix.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190826133837.6784-1-chris@chris-wilson.co.uk