backends/drm: force soft cursor if image creation fails

If the soft cursor is only set but not forced then KWin will try to update
cursors again, making it go into code paths that will crash.

BUG: 445507

src/backends/drm/drm_gpu.cpp

@@ -275,7 +275,7 @@ bool DrmGpu::updateOutputs()
                 } else {
                     auto output = new DrmOutput(conn->pipeline());
                     if (!output->initCursor(m_cursorSize)) {
-                        m_platform->setSoftwareCursor(true);
+                        m_platform->setSoftwareCursorForced(true);
                     }
                     m_drmOutputs << output;
                     m_outputs << output;