i965/mt: Disable HiZ when sharing depth buffer externally (v2)

i965/mt: Disable HiZ when sharing depth buffer externally (v2)

intel_miptree_make_shareable() discarded and disabled CCS. Fix it so
that it discards and disables HiZ too.

Fixes dEQP-EGL.functional.image.render_multiple_contexts.gles2_renderbuffer_depth16_depth_buffer
on Skylake.

v2: Actually do what the commit message says. Discard the HiZ buffer.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=98329
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Nanley Chery <nanley.g.chery@intel.com
Cc: Haixia Shi <hshi@chromium.org>
Cc: mesa-stable@lists.freedesktop.org