backends/drm: fix the incorrect use of std::optional

we only enable an output if we do declare it's enabled = true.

src/backends/drm/drm_backend.cpp

@@ -377,7 +377,7 @@ bool DrmBackend::applyOutputChanges(const OutputConfiguration &config)
             }
             if (const auto changeset = config.constChangeSet(output)) {
                 output->queueChanges(changeset);
-                if (changeset->enabled) {
+                if (changeset->enabled.value_or(output->isEnabled())) {
                     toBeEnabled << output;
                 } else {
                     toBeDisabled << output;

src/core/outputbackend.cpp

@@ -50,7 +50,7 @@ bool OutputBackend::applyOutputChanges(const OutputConfiguration &config)
     QList<Output *> toBeDisabledOutputs;
     for (const auto &output : availableOutputs) {
         if (const auto changeset = config.constChangeSet(output)) {
-            if (changeset->enabled) {
+            if (changeset->enabled.value_or(output->isEnabled())) {
                 toBeEnabledOutputs << output;
             } else {
                 toBeDisabledOutputs << output;