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

replace pipe_mutex_lock() was made unnecessary with fd33a6bcd7.

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>

src/gallium/drivers/r300/r300_blit.c

@@ -328,7 +328,7 @@ static void r300_clear(struct pipe_context* pipe,
             /* Pair the resource with the CMASK to avoid other resources
              * accessing it. */
             if (!r300->screen->cmask_resource) {
-                pipe_mutex_lock(r300->screen->cmask_mutex);
+                mtx_lock(&r300->screen->cmask_mutex);
                 /* Double checking (first unlocked, then locked). */
                 if (!r300->screen->cmask_resource) {
                     /* Don't reference this, so that the texture can be