gallium: split draw_wide_prim stage into separate point/line stages.

gallium: split draw_wide_prim stage into separate point/line stages.

This fixes a validation/code-path problem.  Enabling the stage for the sake
of wide points also inadvertantly caused wide lines to be converted to tris
when we actually want them passed through, such as for the AA line stage.
This is just cleaner now.
Also, replace draw_convert_wide_lines() with draw_wide_line_threshold() as
was done for points.  Allows for 1-pixel lines to be converted too if needed.