drm/i915/selftests: Change priority overflow detection

Check for integer overflow in the priority chain, rather than against a
type-constricted max-priority check.

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/20200519063123.20673-2-chris@chris-wilson.co.uk