spirv: Declare PointCoord as a sysval

Now that all drivers have been patched to convert sysvals to input
varyings when they have too, we can safely declare PointCoord as a sysval
too.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13017>

src/compiler/spirv/vtn_variables.c

@@ -883,8 +883,9 @@ vtn_get_builtin_location(struct vtn_builder *b,
       *location = SYSTEM_VALUE_FRAG_COORD;
       break;
    case SpvBuiltInPointCoord:
-      *location = VARYING_SLOT_PNTC;
       vtn_assert(*mode == nir_var_shader_in);
+      set_mode_system_value(b, mode);
+      *location = SYSTEM_VALUE_POINT_COORD;
       break;
    case SpvBuiltInFrontFacing:
       *location = SYSTEM_VALUE_FRONT_FACE;