drm/i915/execlists: Terminate the context image with BB_END

In the aub trace utility, the context images are terminated with a
MI_BATCH_BUFFER_END; the simulator is reported as complaining otherwise.
Do the same for our protocontext image for completeness, and in passing
apply the magic bit for gen10 to mark the end of the context image.

Reported-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180730164325.12770-1-chris@chris-wilson.co.uk