drm/i915: Assert we idle in the kernel context

Now that we always switch to the kernel context upon idling, we can
make that assertion.

References: 4dfacb0b ("drm/i915: Switch to kernel context before idling at runtime")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180531224057.6036-1-chris@chris-wilson.co.uk