drivers: display: stm32_ltdc: fix return value

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.

"device_is_ready" does check for null, but also for
"dev->state->initialized == false", so we need to isolate the
"dev == NULL" case.

Signed-off-by: Abderrahmane Jarmouni <git@jarmouni.me>