drm/i915: Assert the engine is idle before overwiting the HWS

When we update the global seqno (on the engine timeline), we modify HW
state (both registers and mapped pages). As we do this, we should be
sure that the HW is idle and we are not causing a conflict. The caller
is supposed to wait_for_idle before calling us to update the seqno, so
let's assert they have and the engine is indeed idle.

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

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