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).

src/gallium/docs/source/cso/rasterizer.rst

@@ -83,7 +83,10 @@ point_size_per_vertex
 point_size
     The size of points, if not specified per-vertex.
 point_sprite
-    Whether points are drawn as sprites (textured quads)
+    Whether points are drawn as sprites (textured quads). This is mutually
+    exclusive with point_smooth. Note that sprite_coord_mode set to
+    PIPE_SPRITE_COORD_NONE for all coords and point_sprite enabled is basically
+    equivalent to point_sprite disabled.
 sprite_coord_mode
     Specifies how the value for each shader output should be computed when
     drawing sprites.  If PIPE_SPRITE_COORD_NONE, don't change the vertex
@@ -98,7 +101,7 @@ sprite_coord_mode
     sprite rendering.
     Note that when geometry shaders are available, this state could be
     removed.  A special geometry shader defined by the state tracker could
-    converts the incoming points into quads with the proper texture coords.
+    convert the incoming points into quads with the proper texture coords.
 
 scissor
     Whether the scissor test is enabled.