anv: Add a basic implementation of VK_KHX_external_semaphore

anv: Add a basic implementation of VK_KHX_external_semaphore

This patch adds an implementation based on DRM BOs.  We don't actually
advertise the extension yet because we want to add a couple more paths
first.

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