kernel: clean up zap "shader" handling
Currently the kernel looks for zap firmware, and if it doesn't find it, decides that it is a platform where zap is not required to take the GPU out of secure mode at boot. Unfortunately trying to take GPU out of secure mode directly (without zap) on a platform where zap is required, means insta-reboot.
We should re-work this to refuse to load if zap fw is missing and the zap node in dtb is present. The presence/absence of a zap node is a more reliable way to tell if zap is required to take GPU out of secure mode.