swr: conditionally validate vertex buffer state

swr: conditionally validate vertex buffer state

Vertex buffer state doesn't need to be validated on every call,
only on dirty _NEW_VERTEX or indexed draws.

Unconditional validation was introduced as part of patch 330d0607ed6,
"remove pipe_index_buffer and set_index_buffer", with the expectation
we'd optimize later.

Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>