drm/i915: Assert that the kernel_context is hw-id 0

For easy recognisability, we want the kernel context to have id 0 and
all user contexts to have non-zero ids.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170123113132.18665-1-chris@chris-wilson.co.uk