etnaviv: fold etna_screen_bo_get_handle into etna_resource_get_handle

etnaviv: fold etna_screen_bo_get_handle into etna_resource_get_handle

There is no point in keeping this indirection. Makes the code easier to
follow.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com> (v1)
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>