i965: Fix z_offset computation in intel_miptree_unmap_depthstencil()

i965: Fix z_offset computation in intel_miptree_unmap_depthstencil()

The bug is triggered by using glTexSubImage2d() with GL_DEPTH_STENCIL
as base internal format and non-zero x, y offsets. Currently x, y
offsets are ignored while updating the texture image.

Fixes Khronos GLES3 CTS tests:
npot_tex_sub_image_2d
npot_tex_sub_image_3d
npot_pbo_tex_sub_image_2d
npot_pbo_tex_sub_image_2d

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>