drm/i915: Prevent timeline updates whilst performing reset

As the fence may be signaled concurrently from an interrupt on another
device, it is possible for the list of requests on the timeline to be
modified as we walk it. Take both (the context's timeline and the global
timeline) locks to prevent such modifications.

Fixes: 80b204bc ("drm/i915: Enable multiple timelines")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161223145804.6605-10-chris@chris-wilson.co.uk