Claude review: drm: ensure blend mode supported if pixel format with alpha exposed

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

Patch Review

**Potential NULL pointer dereference.** In `validate_blend_mode_for_alpha_formats()`:

```c
for (i = 0; i < plane->format_count; i++) {
    fmt = drm_format_info(plane->format_types[i]);
    if (fmt->has_alpha) {
```

`drm_format_info()` can return NULL for unknown formats (it only does a `WARN_ON(!info)` internally, then returns NULL). If a buggy driver registers an unknown fourcc, this validation function — which exists precisely to catch driver bugs — would hit a NULL dereference on `fmt->has_alpha`.

This should have a NULL check:

```c
fmt = drm_format_info(plane->format_types[i]);
if (fmt && fmt->has_alpha) {
```

In practice this is unlikely to trigger since any driver with an unknown format would already be very broken, but a validation function should be defensive.

**Otherwise the logic is correct.** The early return when `blend_mode_property` is set avoids unnecessary iteration. The `break` after the first warning avoids spamming the log. The placement in `drm_mode_config_validate()` in the existing `drm_for_each_plane` loop is efficient.

**Minor style nit:** The WARN format string is split across lines:

```c
WARN(1, "[PLANE:%d:%s] pixel format with alpha exposed but "
     "blend mode not setup", plane->base.id, plane->name);
```

"not setup" should be "not set up" (two words — "setup" is a noun, "set up" is the verb/participle form). Very minor.

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Generated by Claude Code Patch Reviewer