Claude review: drm/i915: Implement the "color format" DRM property

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

Patch Review

1. **Dropped `has_hdmi_sink` check in HDMI fallback path:** The existing code checks `!crtc_state->has_hdmi_sink` before falling back to YCbCr420, but the new code drops this:
```c
-		if (crtc_state->sink_format == INTEL_OUTPUT_FORMAT_YCBCR420 ||
-		    !crtc_state->has_hdmi_sink ||
-		    !connector->base.ycbcr_420_allowed ||
-		    !drm_mode_is_420_also(info, adjusted_mode))
+		if (!connector->base.ycbcr_420_allowed)
+			return ret;
```
DVI sinks (non-HDMI) cannot do YCbCr420, but the `ycbcr_420_allowed` flag should already prevent this. However, the original code had the `has_hdmi_sink` check as an additional safety guard. Removing it is probably fine since `ycbcr_420_allowed` should never be set for DVI, but worth double-checking.

2. **`drm_mode_is_420_also` changed to `drm_mode_is_420`:** In the HDMI fallback, the old code checks `drm_mode_is_420_also()` and the new code uses `drm_mode_is_420()` (which includes both 420-only and 420-also). Since we already check `crtc_state->sink_format == INTEL_OUTPUT_FORMAT_YCBCR420` (i.e., we're not already in 420 mode), this is a semantic change but correct — if the mode is 420-only and we're somehow not already in 420, it's still valid to fall back to 420.

3. **Implicit reliance on `!req_fmt` meaning AUTO (value 0):** Throughout the i915 HDMI path, `!req_fmt` and `if (req_fmt)` are used to test for AUTO vs non-AUTO. Since `DRM_CONNECTOR_COLOR_FORMAT_AUTO = 0`, this works, but it's fragile if the enum ever changes. Recommend using explicit comparison: `req_fmt == DRM_CONNECTOR_COLOR_FORMAT_AUTO`.

4. **i915 DP-MST: `actual_format` potentially uninitialized:** In `mst_stream_compute_config`, the switch on `pipe_config->output_format`:
```c
+		switch (pipe_config->output_format) {
+		case INTEL_OUTPUT_FORMAT_RGB:
+			actual_format = DRM_CONNECTOR_COLOR_FORMAT_RGB444;
+			break;
+		case INTEL_OUTPUT_FORMAT_YCBCR444:
+			actual_format = DRM_CONNECTOR_COLOR_FORMAT_YCBCR444;
+			break;
+		case INTEL_OUTPUT_FORMAT_YCBCR420:
+			actual_format = DRM_CONNECTOR_COLOR_FORMAT_YCBCR420;
+			break;
+		}
```
There is no `default:` case. If `pipe_config->output_format` is an unexpected value, `actual_format` will be used uninitialized. The compiler will likely warn about this. Add a `default: return -EINVAL;` case.

5. **No YCbCr422 support in i915:** The `supported_colorformats` bitmask in `intel_connector.c` omits `YCBCR422`. This is correct since i915 doesn't support YCbCr422 output, and the property will correctly not list it.

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