anv: don't leave anv_batch fields undefined
Because the extend_cb vfunc is not initialized, there is a risk that the emission code calls into a random pointer. Signed-off-by:Lionel Landwerlin <lionel.g.landwerlin@intel.com> Cc: mesa-stable Reviewed-by:
Rohan Garg <rohan.garg@intel.com> Part-of: <mesa/mesa!14418> (cherry picked from commit 1d40d53e)