gallium/ttn: Fix the type of gl_FragDepth.

In TGSI we have a vec4 of which only .z is used, but for NIR we should be
using a float the same as other NIR IR.  We were already moving TGSI's .z
to the .x channel.

Acked-by: Rob Clark <robdclark@gmail.com>

src/gallium/auxiliary/nir/tgsi_to_nir.c

@@ -344,6 +344,7 @@ ttn_emit_declaration(struct ttn_compile *c)
                }
                case TGSI_SEMANTIC_POSITION:
                   var->data.location = FRAG_RESULT_DEPTH;
+                  var->type = glsl_float_type();
                   break;
                default:
                   fprintf(stderr, "Bad TGSI semantic: %d/%d\n",