i965: Fix transform feedback of gl_PointSize.

i965: Fix transform feedback of gl_PointSize.

On i965 Gen6 and above, gl_PointSize is stored in component W of the
first VUE slot (which corresponds to VERT_RESULT_PSIZ in the VUE map).
Normally we store varying floats in component X of a VUE slot, so we
need special case logic for gl_PointSize.

For Gen6, we do this with a ".wwww" swizzle in the GS.  For Gen7, we
shift the component mask by 3 to select the W component.

Fixes Piglit test "EXT_transform_feedback/builtin-varyings
gl_PointSize".

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>