drm/i915: Disable RCS flips on Ivybridge

We currently see random GPU hangs when using RCS flips with multiple
pipes on Ivybridge. Now that we have mmio flips, we can fairly cheaply
fallback to using CPU driven flips instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77104


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>