radeon: fix stencil miptree allocation of combined ZS buffers on EG and SI

This allows texturing with depth-stencil buffers directly without the copy
to CB. The separate miptree description for stencil is added, because
the stencil mipmap offsets are not really depth offsets/4 (at least
for the texture units).

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>