drm/i915/execlists: Ignore the inactive kernel context in assert_pending_valid

Filter out warnings for the kernel context that is used to flush
inactive contexts, as they do no not pose a risk.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101082919.21122-1-chris@chris-wilson.co.uk