drm/i915/gt: Move legacy context wa to intel_workarounds

Use the central mechanism for recording and verifying that we restore
the w/a for the older devices as well.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200601072446.19548-3-chris@chris-wilson.co.uk