drm/i915: Workaround incoherence with fence updates on Valleyview

In commit 25ff1195
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Apr 4 21:31:03 2013 +0100

    drm/i915: Workaround incoherence between fences and LLC across multiple CPUs

we introduced an empirical workaround for memory corruption when using
fences from multiple CPUs. At the time, we did not have any results for
Valleyview, so the presumption was that it was limited to recent
generations using LLC. Now we have evidence that Valleyview also suffers
incoherence and requires a similar but different workaround. For
Valleyview, the wbinvd instruction is insufficient and we require the
serialising register write per-CPU. Conversely, that serialising
register write is not enough for SNB/IVB/HSW. To compromise and keep the
code relatively clean, employ both serialisation techniques in the same
workaround.

Reported-by: Jon Bloomfield <jon.bloomfield@intel.com>
Tested-by: Jon Bloomfield <jon.bloomfield@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62191


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