clover: Set PIPE_USAGE_IMMUTABLE for read-only host-ptrs.
This conveys intent to the driver at the time of resource creation for host memories which are physically immutable. In other cases, buffers are almost certainly written to at some point (to get the data to them) so the CL_MEM_* prot flags are relegated to formalities for validation, but in this case it is truly immutable, such as with shared memory pages w/ file-backings, etc.
Related: !16115 (merged)