Claude review: media: iris: don't specify highest_bank_bit in the source code

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

Patch Review

Replaces `core->iris_platform_data->ubwc_config->highest_bank_bit` with direct access to the UBWC config:

```c
payload = ubwc->highest_bank_bit;
```

The old field was `u32`, the new struct field is `int`. Since `highest_bank_bit` values are always positive (representing a DDR bank bit position, e.g. 16 for SM8550), the implicit conversion to `u32 payload` is safe.

The key benefit: the central registry can derive HBB from runtime DDR type detection rather than using a fixed value, which was called out in the cover letter as one of the motivations for this series.

No issues.

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