drm/i915/selftests: Verify LRC isolation

Record the LRC registers before/after a preemption event to ensure that
the first context sees nothing from the second client; at least in the
normal per-context register state.

References: https://gitlab.freedesktop.org/drm/intel/issues/1233Signed-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/20200227085723.1961649-12-chris@chris-wilson.co.uk