drm/i915/bdw: Flush system agent on gen8 also
drm/i915/bdw: Flush system agent on gen8 also

gem_gtt_cpu_tlb seems to indicate that it is needed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72869

Signed-off-by: Ben Widawsky <73675debcd8a436be48ec22211dcf44fe0df0a64@bwidawsk.net>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs