drm/i915/execlists: Add a paranoid flush of the CSB pointers upon reset

After a GPU reset, we need to drain all the CS events so that we have an
accurate picture of the execlists state at the time of the reset. Be
paranoid and force a read of the CSB write pointer from memory.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190912092933.4729-1-chris@chris-wilson.co.uk