RDMA/cxgb4: Fastreg NSMR fixes

- Remove dsgl support - doesn't work in T4.
- Wrap the immediate PBL as needed when building it in the wr.
- Adjust max pbl depth allowed based on ulptx alignment requirements.
- Bump the slots per SQ to 5 to allow up to 128MB fast registers.
- Advertise fastreg support by default.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>