anv: fix calculation of buffer size in case dynamic size is used

VK spec got clarification about the pSizes parameter. Patch also fixes setting up NullVertexBuffer in case dynamic size is in use.

Fixes set of new tests: dEQP-VK.pipeline.extended_dynamic_state*with_offset

Fixes: b9a05447 ("anv: dynamic vertex input binding stride and size support") Signed-off-by: Tapani Pälli tapani.palli@intel.com