xiaomi-tissot: switch to mainline

Adding a mainline kernel. Moving downstream to unmaintained as it's not relevant anymore. Kernel upgrade to 5.15-rc6. Works with 5.16 kernel (Draft until !2960 is merged). The firmware package has also been upgraded. In the kernel config, msm_drm is temporarily marked as a module (It was agreed with ultracoolguy and unknown). This is because msm_drm build-in causes a kernel panic on xiaomi-tissot for an unknown reason (Already done in !2960). Therefore, on some msm8953 devices, it was necessary to enable msm in the initramfs for osk-sdl to work (Changes are not related to this MR, so have been cancelled).

• USB Networking, Flashing, Touchscreen, Display, WiFi, Xwayland, FDE, 3D Acceleration, Bluetooth - Works
• Battery (Because the charger is not working), Audio (Everything except the main speaker works) - Partial
• Accelerometer, Camera, GPS, Modem(Works with patches from another branch), Charger(it will be fixed in 5.16 - 5.17 kernel), USB OTG(Should be fixed along with the addition of the charger), Ir TX - Broken