drm/i915: Acquire the signaler's timeline HWSP last

Acquiring the signaler's timeline takes an active reference to their
HWSP that we would like to avoid if possible, so take it after
performing all of our allocations required to set up the fencing. The
acquisition also provides the final check that the target has not
already signaled allowing us to avoid the semaphore at the last moment.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190503140239.32668-1-chris@chris-wilson.co.uk