drm/i915/selftests: Add a simple rollover test for the kernel context

Exercise the seqno wrap paths on the kernel context to provide a small
amount of sanity checking and ensure that they are visible to lockdep.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204094102.876636-1-chris@chris-wilson.co.uk