hwcontext_vulkan: initialize semaphores of DMABUF imports

hwcontext_vulkan: initialize semaphores of DMABUF imports

There was a recent change in Intel's driver that triggered a driver-internal
error if the semaphore given to the command buffer wasn't initialized.
Given that the specifications require the semaphore to be initialized,
this is within spec. Unlike what's causing it in the first place, which is
that there are no ways to extract/import dma sync objects from DMABUFs,
so we must leave our semaphores bare.