u_blitter: don't fail mipmap generation for depth formats containing stencil

Bugzilla: https://bugzilla.freedesktop.org/show_bug.cgi?id=109754



Cc: 19.0 19.1 <mesa-stable@lists.freedesktop.org>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
(cherry picked from commit 4b11ed44)