kms: Add support for "skip" key in the output config

"off" is not quite suitable because that turns the output off, as the name suggests.
Disconnected outputs are skipped automatically - it is natural to have a way to
request the same for connected outputs as well.

Task-number: QTBUG-74871
Change-Id: I8bea83428ae0424601b19482b6e6ef809491d0fb
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/platformsupport/kmsconvenience/qkmsdevice.cpp

@@ -59,6 +59,7 @@ enum OutputConfiguration {
     OutputConfigOff,
     OutputConfigPreferred,
     OutputConfigCurrent,
+    OutputConfigSkip,
     OutputConfigMode,
     OutputConfigModeline
 };
@@ -191,6 +192,8 @@ QPlatformScreen *QKmsDevice::createScreenForConnector(drmModeResPtr resources,
         configuration = OutputConfigPreferred;
     } else if (mode == "current") {
         configuration = OutputConfigCurrent;
+    } else if (mode == "skip") {
+        configuration = OutputConfigSkip;
     } else if (sscanf(mode.constData(), "%dx%d@%d", &configurationSize.rwidth(), &configurationSize.rheight(),
                       &configurationRefresh) == 3)
     {
@@ -229,6 +232,11 @@ QPlatformScreen *QKmsDevice::createScreenForConnector(drmModeResPtr resources,
         return nullptr;
     }
 
+    if (configuration == OutputConfigSkip) {
+        qCDebug(qLcKmsDebug) << "Skipping output" << connectorName;
+        return nullptr;
+    }
+
     // Get the current mode on the current crtc
     drmModeModeInfo crtc_mode;
     memset(&crtc_mode, 0, sizeof crtc_mode);