i965: Stop storing sdc_offset in brw_stage_state.

i965: Stop storing sdc_offset in brw_stage_state.

sdc_offset is produced and consumed in the same function, so there's no
need to store it in the context, nor pass pointers to it through various
call chains.

Saves 128 bytes per brw_stage_state structure, and makes the code
clearer as well.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>