gpu: nova-core: gsp: fix improper indexing in driver_read_area

gpu: nova-core: gsp: fix improper indexing in driver_read_area

The current code indexes into `after_rx` using `tx` which is an index
for the whole buffer, not the split buffer `after_rx`.

Also add more rigorous no-panic proofs.

Fixes: 75f6b1de8133 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260129-nova-core-cmdq1-v3-5-2ede85493a27@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>