drm/i915: Introduce struct class_instance for engines across the uAPI

SSEU reprogramming of the context introduced the notion of engine class
and instance for a forwards compatible method of describing any engine
beyond the old execbuf interface. We wish to adopt this class:instance
description for more interfaces, so pull it out into a separate type for
userspace convenience.

Fixes: e46c2e99 ("drm/i915: Expose RPCS (SSEU) configuration to userspace (Gen11 only)")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Cc: Tony Ye <tony.ye@intel.com>
Cc: Andi Shyti <andi@etezian.org>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Tony Ye <tony.ye@intel.com>
Reviewed-by: Andi Shyti <andi@etezian.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190412071416.30097-1-chris@chris-wilson.co.uk