drm/i915/gt: Give engine->kernel_context distinct timeline lock classes

Assign a separate lockclass to the perma-pinned timelines of the
kernel_context, such that we can use them from within the user timelines
should we ever need to inject GPU operations to fixup faults during
request construction.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008185941.15228-1-chris@chris-wilson.co.uk