drm/etnaviv: fix DMA direction handling for cached RW buffers

The dma sync operation needs to be done with DMA_BIDIRECTIONAL when
the BO is prepared for both read and write operations.

Fixes: a8c21a54 ("drm/etnaviv: add initial etnaviv DRM driver")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>