drm/i915: Only insert the mb() before updating the fence parameter

With a fence, we only need to insert a memory barrier around the actual
fence alteration for CPU accesses through the GTT. Performing the
barrier in flush-fence was inserting unnecessary and expensive barriers
for never fenced objects.

Note removing the barriers from flush-fence, which was effectively a
barrier before every direct access through the GTT, revealed that we
where missing a barrier before the first access through the GTT. Lack of
that barrier was sufficient to cause GPU hangs.

v2: Add a couple more comments to explain the new barriers

Signed-off-by: Chris Wilson <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Reviewed-by: Jesse Barnes <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>

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