anv/pipeline: Get rid of the no kernel input parameters hack

Previously, meta would pass null shaders in for the VS when it intended to
disable the VS.  However, this meant that we didn't know what inputs we had
and would dead-code things in the FS.  In order to solve this, we
hard-coded a number.  Now meta passes in a VS even if it plans to disable
the stage so this is no longer needed.