drm/i915/bdw: Do not initialize PPGTT in the legacy way for execlists

A pending commit removes synchronous mode from switch_mm.  This breaks
execlists because switch_mm will always try to write to the legacy ring
buffer.

Return immediately from i915_ppgtt_init_gw in execlists mode.
No longer check for execlists mode in gen8_ppgtt_enable() because this
will no longer be called in execlists mode.

Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>