backends/drm: Handle failing to reopen drm node

Otherwise the drm backend will crash.

src/backends/drm/drm_backend.cpp

@@ -204,11 +204,16 @@ DrmGpu *DrmBackend::addGpu(const QString &fileName)
         return nullptr;
     }
 
+    DrmGpu *gpu = new DrmGpu(this, fileName, fd, buf.st_rdev);
+    if (!gpu->graphicsBufferAllocator()) {
+        delete gpu;
+        return nullptr;
+    }
+
     qCDebug(KWIN_DRM, "adding GPU %s", qPrintable(fileName));
-    m_gpus.push_back(std::make_unique<DrmGpu>(this, fileName, fd, buf.st_rdev));
-    auto gpu = m_gpus.back().get();
     connect(gpu, &DrmGpu::outputAdded, this, &DrmBackend::addOutput);
     connect(gpu, &DrmGpu::outputRemoved, this, &DrmBackend::removeOutput);
+    m_gpus.emplace_back(gpu);
     Q_EMIT gpuAdded(gpu);
     return gpu;
 }

src/backends/drm/drm_gpu.cpp

@@ -87,7 +87,9 @@ DrmGpu::DrmGpu(DrmBackend *backend, const QString &devNode, int fd, dev_t device
 
     // Reopen the drm node to create a new GEM handle namespace.
     m_gbmFd = FileDescriptor{open(devNode.toLocal8Bit(), O_RDWR | O_CLOEXEC)};
-    if (m_gbmFd.isValid()) {
+    if (!m_gbmFd.isValid()) {
+        qCCritical(KWIN_DRM) << "Failed to reopen" << devNode << "drm node, expect bad things to happen:" << strerror(errno);
+    } else {
         drm_magic_t magic;
         drmGetMagic(m_gbmFd.get(), &magic);
         drmAuthMagic(m_fd, magic);