gallium: clean up point sprite rasterizer state

gallium: clean up point sprite rasterizer state

Don't need sprite coord origin per coord.
Also, don't need separate sprite enable bit - if all coords have it diabled,
then there are no point sprites (technically, there's a distinction in pre-GL3,
but it only differs in having more leniency in clamping to max size, something
the state tracker would need to handle and the hardware won't bother anyway).
Also, use packed field for the per-coord enables.
All in all, should save 3 dwords in rasterizer state (from 10 down to 7).