drm/i915: fix for when semaphore updates fail

This fixes a long standing issue where emitting the semaphore updates
may have failed, but we've already updated our internal data structure.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>