drm/i915: Use mmio flips to change tiling mode on Baytrail

For whatever reason, MI_DISPLAY_FLIP fails to change tiling mode on
Baytrail, so just use CPU driven mmio flips instead.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76176


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>