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)