backends/drm: fix vblank calculation

Due to some misinterpretation of mode timings, the old method calculated vsync, not vblank,
so the resulting duration was much shorter, which caused frame drops on some systems.

BUG: 482064

src/backends/drm/drm_connector.cpp

@@ -71,7 +71,7 @@ std::shared_ptr<DrmBlob> DrmConnectorMode::blob()
 
 std::chrono::nanoseconds DrmConnectorMode::vblankTime() const
 {
-    return std::chrono::nanoseconds(((m_nativeMode.vsync_end - m_nativeMode.vsync_start) * m_nativeMode.htotal * 1'000'000ULL) / m_nativeMode.clock);
+    return std::chrono::nanoseconds(((m_nativeMode.vtotal - m_nativeMode.vdisplay) * m_nativeMode.htotal * 1'000'000ULL) / m_nativeMode.clock);
 }
 
 drmModeModeInfo *DrmConnectorMode::nativeMode()