drm/i915/bdw: Always use MMIO flips with Execlists

The normal flip function places things in the ring in the legacy
way, so we either fix that or force MMIO flips always as we do in
this patch.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Checkpatch. Fucking again.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>