drivers: display: stm32_ltdc: fix for stm32f429i_disc1

Display is not working on STM32F429i-DISC1 board because
display_blanking_off() needs to be sent to ILI9341 device, but it's sent
to LTDC instead which does not implement it.
This patch adds a LTDC DT property that provides the pHandle of the
display's own controller so that display_blanking_off/on are forwarded to
it when they are called by an application.

Signed-off-by: Abderrahmane Jarmouni <abderrahmane.jarmouni-ext@st.com>