nv50,nvc0: fix 3d blits with mipmap levels

Make sure to normalize the z coordinates as well as the x/y ones when
there are mipmaps present. Fixes 3d mipmap generation, which now uses
the blit path.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>