etnaviv: fill in layer_stride for imported resources

etnaviv: fill in layer_stride for imported resources

The layer stride information is used in various parts of the driver,
so it needs to be present regardless if the driver allocated the
buffer itself or merely imported it from an external source.

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