drm/i915/selftests: Check a few more fixed locations within the context image

As we use hard coded offsets for a few locations within the context
image, include those in the selftests to assert that they are valid.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028121803.29408-1-chris@chris-wilson.co.uk