Fix i915_wait_irq in the presence of interrupt masking.

In the short-circuit code for the breadcrumb already being new enough, we
need to update the sarea_priv copy of the breadcrumb just as if we had
waited.  Otherwise userland error checking will notice that we returned
too early based on its wrong information, and call wait_irq again (leading
to spinning until someone else comes along and updates the sarea_priv).

This bug was hidden when we had interrupt masking disabled, such as in
master, since the interrupt handler would update sarea_priv.