drm/i915: Move engine->needs_cmd_parser to engine->flags

Will be adding a new per-engine flags shortly so it makes sense
to consolidate.

v2: Keep the original code flow in intel_engine_cleanup_cmd_parser.
    (Joonas Lahtinen)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171129082409.18189-1-tvrtko.ursulin@linux.intel.com