drm/i915: Drop the shadow w/a batch buffer

While this is technically the batch as executed by the HW (in part at
least), it is confusing, and only used for a minority of gen.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110123059.1348712-2-chris@chris-wilson.co.uk