gallium/u_blitter: Read MSAA z/s from sampler's .x instead of .y or .z.

u_format defines depth formats as having depth in .x, mesa/st samples for
depth or stencil in .x (not making use of any other channels).
util_make_fs_blit_zs() looks for depth or stencil in .x.  The MSAA path
was the exception looking for it in .z or .y, which was causing drivers to
need to splat their values out to the other channels.  This should be
better on hardware that can emit shorter messages for sampling just the
first channels.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13446>

src/gallium/auxiliary/util/u_simple_shaders.c

@@ -623,7 +623,8 @@ util_make_fs_blit_msaa_depth(struct pipe_context *pipe,
                              enum tgsi_texture_type tgsi_tex)
 {
    return util_make_fs_blit_msaa_gen(pipe, tgsi_tex, "FLOAT",
-                                     "POSITION", ".z", "", "");
+                                     "POSITION", ".z", "",
+                                     "MOV TEMP[0].z, TEMP[0].xxxx\n");
 }
 
 
@@ -637,7 +638,8 @@ util_make_fs_blit_msaa_stencil(struct pipe_context *pipe,
                                enum tgsi_texture_type tgsi_tex)
 {
    return util_make_fs_blit_msaa_gen(pipe, tgsi_tex, "UINT",
-                                     "STENCIL", ".y", "", "");
+                                     "STENCIL", ".y", "",
+                                     "MOV TEMP[0].y, TEMP[0].xxxx\n");
 }