ARM: dts: stm32: Fix display-timings settings for stm32f746-disco

Since commit ef4ce6df "video: stm32: stm32_ltdc: fix data enable polarity"
The panel display output wasn't functional anymore.
Device tree display-timings de-active property value must be updated
to 1.

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