H.264 and H.265 decoding is handled by the VPU, and there was already a successful kickstarter to add mainline support for that: https://www.kickstarter.com/projects/bootlin/allwinner-vpu-s...

Ditto for the HDMI PHY, you can spin up Xorg with a fully libre stack on nearly any of the Allwinner boards from OrangePi, FriendlyARM, etc. The Mali GPU is really only useful for OpenGL ES (aka video games).

Generally the SoCs used will have a separate video encoder IP-block that's not part of the Mali GPU. For example the "Cedar" video engine on Allwinner SoCs, Rockchip's VPU, Samsung's Multi-Function Codec, Qualcomm's Venus.