vc4: Drop VS inputs to 8.
vc4: Drop VS inputs to 8.

In the hardware we only get to declare 8 vertex elements (GLES2's
minimum), so we should be exposing that number here.  Fixes an assertion
failure in piglit texrect-many, at the expense of various GL 2.0-ish
minmax tests now complaining that our count is too low.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler