etnaviv: also update textures from external resources

etnaviv: also update textures from external resources

This reworks the logic in etna_update_sampler_source to select the
newest resource view for updating the texture view. This should make
the logic easier to follow and fixes texture updates from imported
dma-bufs.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>