gallium/util: replace pipe_mutex_init() with mtx_init()

gallium/util: replace pipe_mutex_init() with mtx_init()

pipe_mutex_init() was made unnecessary with fd33a6bcd7f12.

Replace was done using:
find ./src -type f -exec sed -i -- \
's:pipe_mutex_init(\([^)]*\)):(void) mtx_init(\&\1, mtx_plain):g' {} \;

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