drm/i915: Poison the request before emitting commands

If we poison the request before we emit commands, it should be easier to
spot when we execute an uninitialised request.

References: https://bugs.freedesktop.org/show_bug.cgi?id=100144


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170423170619.7156-2-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>