Eh… lk/platform/msm_shared/smem.h has all the definitions in it. Not, mind you, the logic that decides which constants apply.
I think the one in question is HW_PLATFORM_SBC [= 24].
More importantly, that code being pointed out is just deciding whether or not the lk should show a splash screen. Not exactly the most pressing aspect of bringing up a display.
I suggest focusing on the linux kernel side of things first. Get that working, because that is the only part that is actually relevant. The only downside to lk NOT showing a splash screen, is that it takes a bit longer before you actually see that something is happening with the device. Especially since the default kernel ALSO doesn’t show a splash screen, so the first display output is actually from the boot animation AFTER the kernel has finished initializing.