drm/i915: Define an engine class enum for the uABI

We want to be able to report back to userspace details about an engine's
class, and in return for userspace to be able to request actions
regarding certain classes of engines. To isolate the uABI from any
variations between hw generations, we define an abstract class for the
engines and internally map onto the hw.

v2: Remove MAX from the uABI; keep it internal if we need it, but don't
let userspace make the mistake of using it themselves.
v3: s/OTHER/INVALID/
  The use of OTHER is ill-defined, so remove it from the uABI as any
  future new type of engine can define a class to suit it. But keep a
  reserved value for an invalid class, so that we can always
  unambiguously express when something doesn't belong to the
  classification.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> #v2
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110142634.10551-1-chris@chris-wilson.co.uk