vkr: fix external memory validation errors

Merged Chia-I Wu requested to merge olv/virglrenderer:improve-conformance into master

vkcube runs without validation errors after this fix and after mesa/mesa!10709 (merged).

FORCE_ENABLE_DMABUF can still cause validation errors. I believe a new Vulkan extension needs to be created to fully resolve the issues. But that doesn't mean that it can't use some improvements. This is just the first step, to make venus validation error free for simple apps. I will follow up with more changes when I find the time.

/cc @zzyiwei

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