Claude review: drm/v3d: Skip CSD when it has zeroed workgroups

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Patch Review

**Status: Good**

This patch adds a check in `v3d_csd_job_run()` to skip hardware submission when any workgroup dimension is zero. The check is placed after the existing error check and before setting `active_job`:

```c
if (!V3D_GET_FIELD(job->args.cfg[0], V3D_CSD_QUEUED_CFG0_NUM_WGS_X) ||
    !V3D_GET_FIELD(job->args.cfg[1], V3D_CSD_QUEUED_CFG1_NUM_WGS_Y) ||
    !V3D_GET_FIELD(job->args.cfg[2], V3D_CSD_QUEUED_CFG2_NUM_WGS_Z))
	return NULL;
```

The use of `V3D_GET_FIELD` with the proper field macros (bits [31:16] per `v3d_regs.h:416-429`) is the right approach, extracting only the workgroup count bits rather than checking the whole register value. This correctly handles `cfg[]` values that might have non-zero bits in the lower half but a zero workgroup count in the upper half.

The comment explaining the hardware behavior (0 is interpreted as 65536) is accurate and provides good justification for why this check is needed.

The placement before `v3d->queue[V3D_CSD].active_job = &job->base` is correct — by returning `NULL` before setting `active_job`, the function avoids leaving the queue in a state where it thinks a job is active but no fence was created and no hardware submission occurred. This matches the existing pattern used by the error-path check above it (lines 350-353 in the current tree).

The `Fixes` tag references the original CSD support commit, which is appropriate since the zero-workgroup case was always a latent bug — it just wasn't reachable until indirect CSD was added.

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