ir3: use OPC_GETBUF to get size of sampler buffers
The maximum value which
OPC_GETSIZE could return for one dimension
0x007ff0, however sampler buffer could be much bigger.
OPC_GETBUF for them.
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...