drm/i915: Disable MI_SET_CONTEXT psmi w/a for bdw

The current w/a for the gen7 psmi related hangs doesn't apply to bdw, so
disable it if using bdw ringbuffer submission.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324151724.32640-1-chris@chris-wilson.co.uk


Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>