drm/i915: Create distinct lockclasses for execution vs user timelines

In order to simplify the lockdep annotation, as they become more complex
in the future with deferred execution and multiple paths through the
same functions, create a separate lockclass for the user timeline and
the hardware execution timeline.

We should only ever be locking the user timeline and the execution
timeline in parallel so we only need to create two lock classes, rather
than a separate class for every timeline.

v2: Rename the lock classes to be more consistent with other lockdep.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114204105.29171-2-chris@chris-wilson.co.uk