drm/i915/pmu: Drop I915_ENGINE_SAMPLE_MAX from uapi headers

We have agreed during the engine classes discussion that fields marked as
non-ABI are better left out altogether from uapi headers.

v2: Use a local define for maintanability. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123100701.18430-1-tvrtko.ursulin@linux.intel.com