drm/i915: Use LRI to update the semaphore registers

The bspec was recently updated to remove the ability to update the
semaphore using the MI_SEMAPHORE_BOX command, the ability to wait upon
the semaphore value remained. Instead the advice is to update the
register using the MI_LOAD_REGISTER_IMM command. In cursory testing,
semaphores continue to function - the question is whether this fixes
some of the deadlocks where the semaphore registers contained stale
values?

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel J Blueman <daniel@quora.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>