drm/i915: MI_PREDICATE_RESULT_2 is HSW only

The MI_PREDICATE_RESULT_2 register exits only on HSW. On other
platforms the same offset is either reserved, or contains some
other register. So write the register only on HSW.

This regression has been introduced in

commit 9435373e
Author: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Date:   Wed Aug 28 16:45:46 2013 -0300

    drm/i915: Report enabled slices on Haswell GT3

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add regression notice.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>