Claude review: drm/amdgpu: optimize psp_copy_fw()

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

Patch Review

**Correctness: OK**

The original code:
```c
memset(psp->fw_pri_buf, 0, PSP_1_MEG);
memcpy(psp->fw_pri_buf, start_addr, bin_size);
```

is replaced with:
```c
memcpy_and_pad(psp->fw_pri_buf, PSP_1_MEG, start_addr, bin_size, 0);
```

The `memcpy_and_pad` implementation (`lib/string_helpers.c:1004`) does:
```c
if (dest_len > count) {
    memcpy(dest, src, count);
    memset(dest + count, pad, dest_len - count);
} else {
    memcpy(dest, src, dest_len);
}
```

The guard at line 4583 ensures `bin_size <= PSP_1_MEG`, so `memcpy_and_pad` will either copy `bin_size` bytes then zero-pad the remainder (when `bin_size < PSP_1_MEG`), or copy exactly `PSP_1_MEG` bytes (when equal). Both cases are semantically identical to the original. The arguments are mapped correctly: `dest=fw_pri_buf`, `dest_len=PSP_1_MEG`, `src=start_addr`, `count=bin_size`, `pad=0`.

**Nits (commit message only):**

- "1 Mo" should be "1 MB" (Mo is the French abbreviation for megaoctet).
- "the partially re-written" should be "then partially re-written".

These are cosmetic commit message issues; the code change itself is clean and correct.

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