drm/i915/selftests: Improve switch-to-kernel-context checking

We can reduce the switch-to-kernel-context selftest to operate as a loop
and so trivially test another state transition (that of idle->busy).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190307211947.6954-1-chris@chris-wilson.co.uk