anv: ensure prog params are initialized with 0s

anv: ensure prog params are initialized with 0s

As a result of 9baa33cef01f our backend compiler leaves params pretty
much untouched. So in order to avoid storing uninitialized values in
the shader cache blobs, just 0 out this array.

I've considered not even allocating this array which works on gen8+
but the vec4 backend still makes a copy of this array and so it
crashes on memcpy on HSW.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 9baa33cef01f ("anv: Rework push constant handling")
Reported-by: Tapani Pälli <tapani.palli@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Tapani Pälli <tapani.palli@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3516>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3516>