anv: Set up SBE_SWIZ properly for gl_Viewport

gl_Viewport is also in the VUE header so we need to whack the read
offset to 0 and emit a default (no overrides) SBE_SWIZ entry in that
case as well.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

src/intel/vulkan/genX_pipeline.c

@@ -369,8 +369,8 @@ emit_3dstate_sbe(struct anv_pipeline *pipeline)
       if (input_index < 0)
          continue;
 
-      /* gl_Layer is stored in the VUE header */
+      /* gl_Viewport and gl_Layer are stored in the VUE header */
-      if (attr == VARYING_SLOT_LAYER) {
+      if (attr == VARYING_SLOT_VIEWPORT || attr == VARYING_SLOT_LAYER) {
          urb_entry_read_offset = 0;
          continue;
       }