etnaviv: pad scanout buffer size to RS alignment

etnaviv: pad scanout buffer size to RS alignment

This fixes failures to import the scanout buffer with screen resolutions
that don't satisfy the RS alignment restrictions, like 1680x1050.

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