st/mesa: apply DepthMode swizzle to stencil texturing as well

Gallium doesn't present these as GL_RED-style. A swizzle is necessary to
present the proper data in the unused components.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/mesa/state_tracker/st_atom_texture.c

@@ -147,9 +147,7 @@ compute_texture_format_swizzle(GLenum baseFormat, GLenum depthMode,
       else
          return SWIZZLE_XYZW;
    case GL_STENCIL_INDEX:
-      return SWIZZLE_XYZW;
    case GL_DEPTH_STENCIL:
-      /* fall-through */
    case GL_DEPTH_COMPONENT:
       /* Now examine the depth mode */
       switch (depthMode) {