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

pipe_mutex_init() was made unnecessary with fd33a6bcd7.

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>

src/gallium/drivers/vc4/vc4_screen.c

@@ -610,7 +610,7 @@ vc4_screen_create(int fd)
 
         screen->fd = fd;
         list_inithead(&screen->bo_cache.time_list);
-        pipe_mutex_init(screen->bo_handles_mutex);
+        (void) mtx_init(&screen->bo_handles_mutex, mtx_plain);
         screen->bo_handles = util_hash_table_create(handle_hash, handle_compare);
 
         screen->has_control_flow =