softpipe: Don't draw when rasterizer_discard is set
Fixes:
  dEQP-GLES3.functional.rasterizer_discard.basic.write_depth_points
  dEQP-GLES3.functional.rasterizer_discard.basic.write_stencil_points
  dEQP-GLES3.functional.rasterizer_discard.fbo.write_depth_points
  dEQP-GLES3.functional.rasterizer_discard.fbo.write_stencil_points
  dEQP-GLES3.functional.rasterizer_discard.scissor.write_depth_points
  dEQP-GLES3.functional.rasterizer_discard.scissor.write_stencil_points
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>