drm/i915: Increase the RC6p threshold.

This increases GEN6_RC6p_THRESHOLD from 100000 to 150000. For some
reason this avoids the gen6_gt_check_fifodbg.isra warnings and
associated GPU lockups, which makes my ivy bridge machine stable.

Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>