drm/i915/bdw: unleash PPGTT

v2: Squash in fix from Ben: Set PPGTT batches as necessary

This fixes the regression in the last couple of days when we enabled
PPGTT.

v3: Squash in fixup to still use GTT for secure batches from Ville:

BDW doesn't have a separate secure vs. non-secure bit in
MI_BATCH_BUFFER_START. So for secure batches we have to simply
leave the PPGTT bit unset. Fortunately older generations (except
HSW) had similar limitations so execbuffer already creates a GTT
mapping for all secure batches.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>