svga: draw round points when msaa is enabled

svga: draw round points when msaa is enabled

See comments for details.  This allows the piglit
ext_framebuffer_multisample-point-smooth test to pass.

Also, test the pipe_rasterizer_state::point_quad_rasterization field
to see if sprite point rasterization is needed because it's possible
for no sprite_coord_enable bits to be set when drawing sprites.

Finally, remove old, stale comments.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>