iris: Handle vertex shader with window space position
Iris advertises support for PIPE_CAP_TGSI_VS_WINDOW_SPACE_POSITION
so let's actually implement it.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110657
This would fix DX9 games running via gallium9 which use D3DFVF_XYZRHW