gallium/util: replace pipe_mutex_lock() with mtx_lock()

gallium/util: replace pipe_mutex_lock() with mtx_lock()

replace pipe_mutex_lock() was made unnecessary with fd33a6bcd7f12.

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

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