drm/i915: Apply a mb between emitting the request and hangcheck

Seal the request and mark it as pending execution before we submit it to
hardware. We assume that the actual submission cannot fail (that
guarantee is provided by preallocating space in the request for the
submission). As we may inspect this state without holding any locks
during hangcheck we should apply a barrier to ensure that we do
not see a more recent value in the HWS than we are tracking.

Based on a patch by Mika Kuoppala.

Suggested-by: Mika Kuoppala <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Mika Kuoppala <[email protected]>

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