drm/i915/gt: Avoid multi-LRI on Sandybridge

Sandybridge is the gen that didn't handle multiple registers in a single
LRI packet. Don't forget it!

Fixes: 902eb748

 ("drm/i915/gt: Tidy up full-ppgtt on Ivybridge")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217091328.3093551-1-chris@chris-wilson.co.uk