Claude review: drm/drm_blend: allow blend mode property without PREMULTI

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

Patch Review

**Verdict: Looks good, minor nit.**

The core change is clean and well-motivated. The WARN_ON condition is simplified to only reject invalid bits, and a fallback default selection is added:

```c
if (supported_modes & BIT(DRM_MODE_BLEND_PREMULTI))
    default_mode = DRM_MODE_BLEND_PREMULTI;
else if (supported_modes & BIT(DRM_MODE_BLEND_COVERAGE))
    default_mode = DRM_MODE_BLEND_COVERAGE;
else
    default_mode = DRM_MODE_BLEND_PIXEL_NONE;
```

This fallback order is reasonable — PREMULTI first (backward compat), then COVERAGE, then PIXEL_NONE.

**Minor nit:** There is no check that `supported_modes != 0`. Passing zero would create a property with zero enum values and set `default_mode = DRM_MODE_BLEND_PIXEL_NONE` (which isn't even in the enum list). The old WARN_ON would have caught this since `BIT(DRM_MODE_BLEND_PREMULTI) == 0` evaluates true. Consider adding `!supported_modes` to the WARN_ON:

```c
if (WARN_ON(!supported_modes || (supported_modes & ~valid_mode_mask)))
    return -EINVAL;
```

**Documentation update is good** — the `@supported_modes` doc now accurately reflects the new behavior.

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Generated by Claude Code Patch Reviewer