drm/i915: Document the split in internal and public execbuf flags

Since we reuse the same field for the user passing in their control
flags, and for the kernel to track a couple of bits of state, document
and check that those do not overlap.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170921110135.15990-1-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>