This effectively reverts commit 43a171878bb4b5aedb36a.  Technically,
VK_KHR_get_memory_requirements2 and VK_KHR_dedicated_allocation are
required for the KHR version but this at least restores the removed
functionality.  This patch builds but has received zero testing.

Acked-by: Dave Airlie <airlied@redhat.com>