drm/i915: Flush the CSB pointer reset

The HW resets it CSB tail pointer on resetting the engine. Most of the
time. In case it doesn't (and for system resume) we write the expected
value anyway. For extra paranoia, flush the write before we invalidate
the cacheline.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190412110159.10495-1-chris@chris-wilson.co.uk