backends/drm: fix an edge case for DrmGpu::waitIdle
Polling for drm events works for waiting on pageflip events, but if there's commits scheduled for future submission, and that fails, no pageflip will ever arrive. To fix that, this commit ensures at least one commit is actually submitted to the kernel before polling for pageflip events.
(cherry picked from commit 33ad107f)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com