configs: stm32f769-disco: Enable VIDEO_LOGO flag

The patch removes the legacy mode of displaying the ST logo and adopts
the approach introduced by the commit 284b08fb ("board: stm32mp1: add
splash screen with stmicroelectronics logo").

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>