gallium/util: replace pipe_mutex_destroy() with mtx_destroy()

gallium/util: replace pipe_mutex_destroy() with mtx_destroy()

pipe_mutex_destroy() was made unnecessary with fd33a6bcd7f12.

Replace was done with:
find ./src -type f -exec sed -i -- \
's:pipe_mutex_destroy(\([^)]*\)):mtx_destroy(\&\1):g' {} \;

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