i915g: Fix writing of undefined depth value if not writing any outputs.

This was showing up as flakes in CI on tests that didn't write any outputs
but did check the depth result.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10874>

src/gallium/drivers/i915/ci/piglit-i915-g33-flakes.txt

@@ -1,2 +1 @@
 spec@glsl-1.10@execution@discard@glsl-fs-discard-03
-spec@glsl-1.10@glsl-fs-discard-only

src/gallium/drivers/i915/i915_fpc_translate.c

@@ -1187,7 +1187,7 @@ static void
 i915_fixup_depth_write(struct i915_fp_compile *p)
 {
    /* XXX assuming pos/depth is always in output[0] */
-   if (p->shader->info.output_semantic_name[0] == TGSI_SEMANTIC_POSITION) {
+   if (p->shader->info.num_outputs != 0 && p->shader->info.output_semantic_name[0] == TGSI_SEMANTIC_POSITION) {
       const uint depth = UREG(REG_TYPE_OD, 0);
 
       i915_emit_arith(p,