drm/i915: Live testing for context execution

Check we can create and execution within a context.

v2: Write one set of dwords through each context/engine to exercise more
contexts within the same time period.

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/20170213171558.20942-38-chris@chris-wilson.co.uk