Claude review: amdgpu/display/dc/link: fix sparse warning on assigning int as nullptr

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Status: Correct and appropriate.**

The patch changes:
```c
-	struct dc_stream_state *link_stream = 0;
+	struct dc_stream_state *link_stream = NULL;
```

This is a straightforward fix. Assigning `0` to a pointer is valid C but triggers a sparse warning because sparse distinguishes between integer zero and the `NULL` pointer constant. Using `NULL` is the idiomatic and preferred style in the kernel.

**Minor nits on the commit message:**

1. The subject prefix uses `amdgpu/display/dc/link:` but the conventional prefix for this subsystem in the kernel log would be `drm/amd/display:` or `drm/amd/display/dc/link:`. Looking at recent commits for this file, the `drm/amd/` prefix is standard. This should ideally be `drm/amd/display: fix sparse warning ...`.

2. The commit message body says "nullptr" which is a C++ term — the kernel uses C, where it's simply `NULL`. The body could be slightly more precise, e.g., "Fix sparse warning about assigning plain 0 to pointer variable; use NULL instead."

3. The commit message body lacks a `Fixes:` tag identifying when the offending code was introduced. While not strictly required for a style/warning fix, it can be helpful.

None of these are blocking issues — the code change itself is clearly correct and trivially safe.

**Reviewed-by: Acceptable.**

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