i965/miptree: Allocate mt earlier in update winsys

i965/miptree: Allocate mt earlier in update winsys

Later commits require intel_update_image_buffer() to have control over
the miptree creation.   However, intel_update_winsys_renderbuffer_miptree()
currently  creates it based on the given buffer object. This patch moves
the creation to the caller side.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>