st/readpixels: fix accel path for skipimages.

We don't need to use the 3d image address here as that will
include SKIP_IMAGES, and we are only blitting a single
2D anyways, so just use the 2D path.

This fixes some memory overruns under CTS
 packed_pixels.packed_pixels_pixelstore when PACK_SKIP_IMAGES
is used.

Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 6a3e1fb958)

src/mesa/state_tracker/st_cb_readpixels.c

@@ -210,9 +210,9 @@ st_readpixels(struct gl_context *ctx, GLint x, GLint y,
       GLuint row;
 
       for (row = 0; row < (unsigned) height; row++) {
-         GLvoid *dest = _mesa_image_address3d(pack, pixels,
+         GLvoid *dest = _mesa_image_address2d(pack, pixels,
                                               width, height, format,
-                                              type, 0, row, 0);
+                                              type, row, 0);
          memcpy(dest, map, bytesPerRow);
          map += tex_xfer->stride;
       }