anv: Disable VK_KHR_16bit_storage

anv: Disable VK_KHR_16bit_storage

The testing for this extension is currently very poor.  The CTS tests
only test accessing UBOs and SSBOs at dynamic offsets so none of our
constant-offset paths get triggered at all.  Also, there's an assertion
in our handling of nir_intrinsic_load_uniform that offset % 4 == 0 which
is never triggered indicating that nothing every gets loaded from an
offset which is not a dword.  Both push constants and the constant
offset pull paths are complex enough, we really don't want to ship
without tests.  We'll turn the extension back on once we have decent
tests.