radeon/vce: use nblk_y instead of npix_y

radeon/vce: use nblk_y instead of npix_y

npix_y will be removed. level[0].npix_y will be removed too. nblk_y should
be the same as npix_y if the block height == 1. However, nblk_y is aligned
to the tile size, so it can be greater than npix_y.

If that's a problem, we'll have to save the input height of surface_init
and use that.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>