display: stm32-ltdc: incorrect return value for blanking on/off calls when panel controller is not passed in DT

[ajarmouni-st]

Describe the bug
Blanking On/Off calls should not return 0 when there is no panel controller to forward them to, instead they should return ENOSYS to signal to the application that they were not actually executed.

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? stm32f429i_disc1
• What have you tried to diagnose or workaround this issue? N/A
• Is this a regression? No
• ...

To Reproduce
Steps to reproduce the behavior:
1/ remove panel controller from LTDC node

--- a/boards/st/stm32f429i_disc1/stm32f429i_disc1.dts
+++ b/boards/st/stm32f429i_disc1/stm32f429i_disc1.dts
@@ -235,7 +235,6 @@
                     &ltdc_hsync_pc6 &ltdc_vsync_pa4>;
        pinctrl-names = "default";
        ext-sdram = <&sdram2>;
-       panel-controller = <&ili9341>;
        status = "okay";
 
        width = <240>;

2/ west build -p always -b stm32f429i_disc1 samples/drivers/display
3/ west flash
4/ display will fail to work silently (stays white)

Expected behavior
Error message saying that blanking on/off wasn't executed

Impact
Without clear error, will be difficult to debug & fix on any out-of-tree STM32 board that implements display support in the same way as stm32f429i_disc1

Logs and console output
N/A

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows)
• Toolchain (e.g Zephyr SDK, ...)
• Commit v3.7.0-rc2

Additional context
N/A