drm/i915/selftests: Teach switch_to_context() to use the context

The context details which engines to use, so use the ctx->engines[] to
generate the requests to cause the context switch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022130221.20644-2-chris@chris-wilson.co.uk