i965: Eliminate brw->gs.prog_data pointer.

i965: Eliminate brw->gs.prog_data pointer.

Just say no to:

-   brw->gs.base.prog_data = &brw->gs.prog_data->base.base;

We'll just use the brw_stage_prog_data pointer in brw_stage_state
and downcast it to brw_gs_prog_data as needed.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arcero@collabora.com>