svga: disable rasterization if rasterizer_discard is set or FS undefined

svga: disable rasterization if rasterizer_discard is set or FS undefined

With this patch, rasterization will be disabled if the
rasterizer_discard flag is set or the fragment shader
is undefined due to missing position output from the
vertex/geometry shader.

Tested with piglit test glsl-1.50-geometry-primitive-id-restart.
Also tested with full MTT glretrace and piglit.

v2: As suggested by Roland, to properly disable rasterization, besides
    setting FS to NULL, we will also need to disable depth and stencil test.

v3: As suggested by Brian, set SVGA_NEW_DEPTH_STENCIL_ALPHA dirty bit
    in svga_bind_rasterizer_state() if the rasterizer_discard flag is
    changed.

Reviewed-by: Brian Paul <brianp@vmware.com>