drm/i915: Only apply legacy PDE overflow detection to 3lvl machines

Prevent the overflow check from firing on machines with the full 4lvl
page tables, that are not restricted to GEN8_LEGACY_PDES.

v2: Also fix the off-by-one in the compare

Fixes: 894ccebe ("drm/i915: Micro-optimise gen8_ppgtt_insert_entries()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170217141455.19877-1-chris@chris-wilson.co.uk


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