Currently Mali kernel drivers are not implemented into Linux-3.14.29 sources. The buildroot package from Amlogic contains older mali and mali_drm sources delivered outside of kernel sources. Moreover sources from Amlogic has only meson6 meson8 implementations but doesn't have gxbb, gxl, gxm implementations for S905, S905X, and S912 chips correspondingly.
The Hardkernel sources has only gxbb implementation.
The question is that do you have kernel-mode drivers adopted for S905X Khadas VIM?
If no then please ask Amlogic about sourses and we will implement needed drivers into your Linux-3.14.29!
Please NOTE that I talk about sources http://malideveloper.arm.com/resources/drivers/open-source-mali-utgard-gpu-linux-kernel-drivers/
which should be adopted for Linux-3.14.29 sources which we have on your ubuntu branch.