drm/i915/lrc: Prevent preemption when lite-restore is disabled

When WaEnableForceRestoreInCtxtDescForVCS is required, it is only
safe to send new contexts if the last reported event is "active to
idle". Otherwise the same context can fully preempt itself because
lite-restore is disabled.

Testcase: igt/gem_concurrent_blit
Reported-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Tested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>