etnaviv: fill in modifier in etna_resource_get_handle

This allows the state trackers to know the tiling layout of the
resource and pass this through the various userspace protocols.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>