platforms/drm: replace pointer to egl backend with QPointer

If the egl backend gets deleted this leaves a dangling pointer. In
order to prevent crashes with QPainter or EglStreams also don't try
to create dmabuf textures without a EglGbmBackend.

src/plugins/platforms/drm/drm_backend.cpp

@@ -648,13 +648,12 @@ QString DrmBackend::supportInformation() const
 DmaBufTexture *DrmBackend::createDmaBufTexture(const QSize &size)
 {
 #if HAVE_GBM
-    // as the first GPU is assumed to always be the one used for scene rendering
-    // make sure we're on the right context:
-    m_gpus.at(0)->eglBackend()->makeCurrent();
-    return GbmDmaBuf::createBuffer(size, m_gpus.at(0)->gbmDevice());
-#else
-    return nullptr;
+    if (primaryGpu()->eglBackend() && primaryGpu()->gbmDevice()) {
+        primaryGpu()->eglBackend()->makeCurrent();
+        return GbmDmaBuf::createBuffer(size, primaryGpu()->gbmDevice());
+    }
 #endif
+    return nullptr;
 }
 
 DrmGpu *DrmBackend::primaryGpu() const

src/plugins/platforms/drm/drm_gpu.h

@@ -13,6 +13,7 @@
 #include <qobject.h>
 #include <QVector>
 #include <QSocketNotifier>
+#include <QPointer>
 
 #include <epoxy/egl.h>
 
@@ -116,7 +117,7 @@ private:
     void tryAMS();
 
     DrmBackend* const m_backend;
-    AbstractEglBackend *m_eglBackend;
+    QPointer<AbstractEglBackend> m_eglBackend;
 
     const QString m_devNode;
     QSize m_cursorSize;