st/mesa: skip texture validation logic when nothing has changed

st/mesa: skip texture validation logic when nothing has changed

Based on the same logic in the i965 driver 2f225f61451abd51 and
16060c5adcd4.

perf reports st_finalize_texture() going from 0.60% -> 0.16% with
this change when running the Xonotic benchmark from PTS.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>