nine: fix incorrect calculation of layer count

Take into account a number of levels. Simmiliar to how it's done in mesa state tracker.

This fixes crash in iris:

../src/gallium/drivers/iris/iris_resolve.c:670: miptree_layer_range_length: Assertion `start_layer + num_layers <= total_num_layers' failed.

Crash reproducer: https://gitlab.freedesktop.org/yevhenii.kolesnikov/nine-crash-repro