anv: Use BO fences/semaphores for AcquireNextImage

anv: Use BO fences/semaphores for AcquireNextImage

Instead of doing a dummy submit on the command buffer for the fence or a
dummy semaphore and trusting in implicit sync, this commit moves us to
take advantage of implicit sync and just use the WSI image BO as the
fence.  Both semaphores and fences require a tiny bit of extra plumbing
to do this but the result is that we can get rid of a bunch of the extra
synchronization we're doing today.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>