etnaviv: use dummy RT buffer when rendering without color buffer

etnaviv: use dummy RT buffer when rendering without color buffer

At least GC2000 seems to push some dirt from the PE color cache into
the last bound render target when drawing depth only. Newer cores
seem to behave properly and don't do this, but I have found no way
to fix it on GC2000. Flushes and stalls don't seem to make any
difference.

In order to stop the core from pushing the dirt into a precious real
render target, plug in dummy buffer when rendering without a color
buffer.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>