anv: move force shading rate writes checks

anv: move force shading rate writes checks

With variable fragment shading rate, the last pre-rasterization stage
is responsible to write the shading rate value.

The current checks is as follow :

   If the fragment shader can be dispatched at variable shading rate,
   look for the last pre-raster stage to force the write.

We change this to :

   If we're the last pre-raster stage, force the write.

That way this works for pre-rasterization shaders compiled without a
fragment shader.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15637>