Do not try to create a gbm dmabuf when gbm is not available

Allows us to take the fallback path instead of just crashing

plugins/platforms/drm/gbm_dmabuf.cpp

@@ -31,6 +31,10 @@ GbmDmaBuf::~GbmDmaBuf()
 
 KWin::GbmDmaBuf *GbmDmaBuf::createBuffer(const QSize &size, gbm_device *device)
 {
+    if (!device) {
+        return nullptr;
+    }
+
     auto bo = gbm_bo_create(device, size.width(), size.height(), GBM_BO_FORMAT_ARGB8888, GBM_BO_USE_RENDERING | GBM_BO_USE_LINEAR);
 
     if (!bo) {