mesa: Add _NEW_RASTERIZER_DISCARD as synonym for _NEW_TRANSFORM.

mesa: Add _NEW_RASTERIZER_DISCARD as synonym for _NEW_TRANSFORM.

This makes it easier to keep track of which dirty bits correspond to
which pieces of context, since it makes _NEW_RASTERIZER_DISCARD
correspond with ctx->RasterDiscard.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>