drm/i915/trace: Describe engines as class:instance pairs

Instead of using the engine->id, use uabi_class:instance pairs in trace-
points including engine info.

This will be more readable, more future proof and more stable for
userspace consumption.

v2:
 * Use u16 for class and instance. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: svetlana.kukanova@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525082642.18246-1-tvrtko.ursulin@linux.intel.com