drm/i915: Align engine->uabi_class/instance with i915_drm.h

In our ABI we have defined I915_ENGINE_CLASS_INVALID_NONE and
I915_ENGINE_CLASS_INVALID_VIRTUAL as negative values which creates
implicit coupling with type widths used in, also ABI, struct
i915_engine_class_instance.

One place where we export engine->uabi_class
I915_ENGINE_CLASS_INVALID_VIRTUAL is from our our tracepoints. Because the
type of the former is u8 in contrast to u16 defined in the ABI, 254 will
be returned instead of 65534 which userspace would legitimately expect.

Another place is I915_CONTEXT_PARAM_ENGINES.

Therefore we need to align the type used to store engine ABI class and
instance.

v2:
 * Update the commit message mentioning get_engines and cc stable.
   (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 6d06779e ("drm/i915: Load balancing across a virtual engine")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v5.3+
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200116134508.25211-1-tvrtko.ursulin@linux.intel.com