drm/i915: Restore the kernel context after verifying the w/a

As a safety net, flush the engine verifications and restore the kernel
context.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/971


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200126194618.2131078-1-chris@chris-wilson.co.uk