After building the mainline Linux tree, run (where ~/linux is the location of
the kernel source code; more flags needed for cross-compiling?):
$ make KDIR=~/linux -j4
When this completes, there will be a file located at:
driver/product/kernel/drivers/gpu/arm/midgard/mali_kbase.ko
Copy that file to the target running the mainline tree, and run (as root on the target):
# insmod mali_kbase.ko
# chmod 666 /dev/mali0