drm/etnaviv: add function to construct MMUv2 init buffer

Both the safe/scratch address and the master TLB address are per pipe
with the CPU mapped registers not properly propagating to the
different translation units.

The only way to correctly configure all translation units is to have
a command stream snipped executed by the FE, before any other execution
can start.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>