ir3: use OPC_GETBUF to get size of sampler buffers

The maximum value which OPC_GETSIZE could return for one dimension is 0x007ff0, however sampler buffer could be much bigger. Blob uses OPC_GETBUF for them.

Fixes tests:

 dEQP-VK.memory.pipeline_barrier.transfer_dst_uniform_texel_buffer.1048576

After 0x007ff0 GETSIZE starts to return 1. To be honest, I'm not completely sure that it is a limit of GETSIZE and not something else. But OPC_GETBUF returns a correct value, so...