backends/drm: use max frame average brightness instead of max brightness in HDR metadata

Using the peak maximum brightness can cause displays to do more agressive tone mapping, which
overall darkens the image without any reason. Instead, send the max fame average brightness,
which the display can always do over the whole screen

src/backends/drm/drm_pipeline.cpp

@@ -757,11 +757,11 @@ std::shared_ptr<DrmBlob> DrmPipeline::createHdrMetadata(NamedTransferFunction tr
             },
             .white_point = {to16Bit(colorimetry.white.x()), to16Bit(colorimetry.white.y())},
             // in nits
-            .max_display_mastering_luminance = uint16_t(std::round(metadata.desiredContentMaxLuminance)),
+            .max_display_mastering_luminance = uint16_t(std::round(metadata.desiredMaxFrameAverageLuminance)),
             // in 0.0001 nits
             .min_display_mastering_luminance = uint16_t(std::round(metadata.desiredContentMinLuminance * 10000)),
             // in nits
-            .max_cll = uint16_t(std::round(metadata.desiredContentMaxLuminance)),
+            .max_cll = uint16_t(std::round(metadata.desiredMaxFrameAverageLuminance)),
             .max_fall = uint16_t(std::round(metadata.desiredMaxFrameAverageLuminance)),
         },
     };