The kernel device drivers for ARM's current Mali GPUs are available from malideveloper.com under GPLv2. The same goes for their drm driver (separate from their gpu driver) and their UMP device driver. (A device driver for allocating and sharing physically contiguous memory buffers. Embedded GPUs usually don't have their own memory.)
Their EXA and DRI drivers are available under MIT license and their Android gralloc implementation is available under Apache license.
It is only their user space libraries (OpenGL ES 1/2, OpenVG, EGL..) that are under proprietary license.
Ok great. Mesa should replace the user space libs anyway, right? Even tough it's not that insanely optimized, right?
Well.. The kernel module doesn't do much.. It receives command buffers and execute them on the relevant hardware and maps the memory buffers with the data structures it needs, but the generation of these data structures and command queues is done in the user space driver. That is highly hardware specific and adding this functionality to mesa is not a trivial task.