I will recommend first to read this wiki article:
https://wiki.st.com/stm32mpu/index.php/Which_Package_better_suits_your_needs
It will give the different information about what you can do with the different packages delivered with STM32MP15 release and also what you can actually do with it.
Then:
https://wiki.st.com/stm32mpu/index.php/OpenSTLinux_distribution
It will show you the different flavors of openstlinux distribution .
Now, to answer precisely to your question:
STM32MP15 Embedded SW delivery is there to propose an open solution for a general purpose MPU dedicated to the mass market.
Main component is the BSP (Kernel, u-boot, tf-a, optee) and STMicro has added on top of it a framework based on Weston/wayland (that makes sense for this platform especially for our GPU).
You are obviously free to use what makes sense for you (no distro Yocto standard Distro maybe good for your case) or to build your own distribution based on openstlinux BSP.
The SDK you are referring was built for our reference image (weston/wayland one) to be able to rebuild any part of the BSP or any application based on this framework.
If you want to interface directly to kernel display interface (drm/kms), first build your own distribution with yocto (or one of the openstlinux reference), then generate your own SDK based on your distribution (with bitbake).
