freedreno/a3xx: use the same layer size for all slices

freedreno/a3xx: use the same layer size for all slices

We only program in one layer size per texture, so that means that all
levels must share one size. This makes the piglit test

bin/texelFetch fs sampler2DArray

have the same breakage as its non-array version instead of being
completely off, and makes

bin/ext_texture_array-gen-mipmap

start passing.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.4 10.5" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Rob Clark <robclark@freedesktop.org>