etnaviv: enable full overwrite when no color buffer is present

The OVERWRITE bit disables destination fetches, which is exactly what
we want when there is no valid color buffer bound.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>
2 files changed