gallium/util: replace pipe_mutex_unlock() with mtx_unlock()

gallium/util: replace pipe_mutex_unlock() with mtx_unlock()

pipe_mutex_unlock() was made unnecessary with fd33a6bcd7f12.

Replaced using:
find ./src -type f -exec sed -i -- \
's:pipe_mutex_unlock(\([^)]*\)):mtx_unlock(\&\1):g' {} \;

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