anv: support fd=-1 in ImportSemaphoreFd
ImportSemaphoreFdKHR might be called with an fd==-1. If that happens, intead of importing it from an fd, an already signaled syncobj must be created.
The fd could be -1 in certain conditions. For example, AcquireImageANDROID could receive an -1 fd, and pass the fd to ImportSemaphoreFdKHR. Although ATM implicit-sync is being used in *ANDROID.