backends/drm: force a modeset after tty switches with legacy

We don't know what the current mode is, or if the display is even enabled
or whatever. Always do a modeset to be sure everything is correct

BUG: 469280

src/backends/drm/drm_gpu.cpp

@@ -702,6 +702,12 @@ void DrmGpu::setActive(bool active)
             }
             // while the session was inactive, the output list may have changed
             m_platform->updateOutputs();
+            for (const auto &output : std::as_const(m_drmOutputs)) {
+                // force a modeset with legacy, we can't reliably know if one is needed
+                if (!atomicModeSetting()) {
+                    output->pipeline()->forceLegacyModeset();
+                }
+            }
         } else {
             for (const auto &output : std::as_const(m_drmOutputs)) {
                 output->renderLoop()->inhibit();

src/backends/drm/drm_pipeline.h

@@ -72,6 +72,7 @@ public:
     Error present();
     bool testScanout();
     bool maybeModeset();
+    void forceLegacyModeset();
 
     bool needsModeset() const;
     void applyPendingChanges();

src/backends/drm/drm_pipeline_legacy.cpp

@@ -43,6 +43,11 @@ DrmPipeline::Error DrmPipeline::presentLegacy()
     return Error::None;
 }
 
+void DrmPipeline::forceLegacyModeset()
+{
+    legacyModeset();
+}
+
 DrmPipeline::Error DrmPipeline::legacyModeset()
 {
     uint32_t connId = m_connector->id();