backends/drm: always expose brightness control

This allows the user to change the brightness level of content even if there's no
actual underlying "backlight" device. This is the case with many internal OLED
screens for example.

BUG: 413451

src/backends/drm/drm_output.cpp

@@ -236,7 +236,7 @@ static const bool s_allowColorspaceIntel = qEnvironmentVariableIntValue("KWIN_DR
 
 Output::Capabilities DrmOutput::computeCapabilities() const
 {
-    Capabilities capabilities = Capability::Dpms | Capability::IccProfile;
+    Capabilities capabilities = Capability::Dpms | Capability::IccProfile | Capability::BrightnessControl;
     if (m_connector->overscan.isValid() || m_connector->underscan.isValid()) {
         capabilities |= Capability::Overscan;
     }
@@ -261,9 +261,6 @@ Output::Capabilities DrmOutput::computeCapabilities() const
         // TODO only set this if an orientation sensor is available?
         capabilities |= Capability::AutoRotation;
     }
-    if (m_brightnessDevice || m_state.highDynamicRange) {
-        capabilities |= Capability::BrightnessControl;
-    }
     return capabilities;
 }
 
@@ -417,7 +414,6 @@ void DrmOutput::applyQueuedChanges(const std::shared_ptr<OutputChangeSet> &props
     next.desiredModeRefreshRate = props->desiredModeRefreshRate.value_or(m_state.desiredModeRefreshRate);
     setState(next);
 
-    updateInformation();
     if (m_brightnessDevice) {
         if (m_state.highDynamicRange) {
             m_brightnessDevice->setBrightness(1);
@@ -449,7 +445,6 @@ void DrmOutput::setBrightnessDevice(BrightnessDevice *device)
             device->setBrightness(m_state.brightness);
         }
     }
-    updateInformation();
 }
 
 void DrmOutput::revertQueuedChanges()
@@ -504,7 +499,7 @@ QVector3D DrmOutput::effectiveChannelFactors() const
     QVector3D adaptedChannelFactors = Colorimetry::fromName(NamedColorimetry::BT709).toOther(m_state.colorDescription.containerColorimetry()) * m_channelFactors;
     // normalize red to be the original brightness value again
     adaptedChannelFactors *= m_channelFactors.x() / adaptedChannelFactors.x();
-    if (m_state.highDynamicRange) {
+    if (m_state.highDynamicRange || !m_brightnessDevice) {
         // enforce a minimum of 25 nits for the reference luminance
         constexpr double minLuminance = 25;
         const double brightnessFactor = (m_state.brightness * (1 - (minLuminance / m_state.referenceLuminance))) + (minLuminance / m_state.referenceLuminance);