vulkan: optimize UMA buffer operations and fix driver hangs (llama/16059)

vulkan: optimize UMA buffer operations and fix driver hangs (llama/16059)

* vulkan: optimize UMA buffer operations and fix driver hangs

The previous implementation was blocking the GPU for extended periods,
causing the i915 driver to reset the context due to the hangcheck
protection.

[32628.443070] i915 0000:00:02.0: [drm] GPU HANG: ecode 12:1:85dffffb, in llama-server [194114]
[32628.443091] i915 0000:00:02.0: [drm] llama-server[194114] context reset due to GPU hang

* vulkan: implement deferred_memset on UMA

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Signed-off-by: Giuseppe Scrivano <redacted>