drm/i915/selftests: Ignore autoincrementing timestamp on verfifying whitelists

As a timestamp will automatically update itself, it will not hold only
contexts we write into it, and will change from the baseline value
making us suspect that our writes are landing. As this confuses us and
we would need more careful treatment to detect invalid stores into the
timestamp, skip it when verifying the whitelists.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200602154839.6902-1-chris@chris-wilson.co.uk