zink: Initialize zink_sampler->batch_uses on creation.

Otherwise sampler->batch_uses.usage, might be used uninitialised in

==468944==    at 0xBD216EB: ptr_add_usage (zink_batch.c:370)
==468944==    by 0xBD218CD: zink_batch_reference_sampler (zink_batch.c:411)
==468944==    by 0xBCFFBBD: update_sampler_descriptors (zink_descriptors.c:1078)
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