configs: stm32f746-disco: limit resolution to 480x272

The patch fixes the y-resolution, which was causing the creation of a
framebuffer larger than actually needed, resulting in memory waste.

Fixes: cc1b0e7b ("board: Add display to STM32F746 SoC discovery board")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>