drm/i915: Protect timeline->hwsp dereferencing

As not only is the signal->timeline volatile, so will be acquiring the
timeline's HWSP. We must first carefully acquire the timeline from the
signaling request and then lock the timeline. With the removal of the
struct_mutex serialisation of request construction, we can have multiple
timelines active at once, and so we must avoid using the nested mutex
lock as it is quite possible for both timelines to be establishing
semaphores on the other and so deadlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190919111912.21631-3-chris@chris-wilson.co.uk