util: _debug_printf should print even when DEBUG is not defined

The leading underscore is meaningful...  This function is used by
_warning and _error functions as well as the more common
debug_printf().

debug_printf (without underscore) gets turned off when DEBUG is
disabled, but warning/error messages still use this function to get
their message out.
(cherry picked from commit 0ac879dca7)

src/gallium/auxiliary/util/u_debug.c

@@ -143,11 +143,9 @@ void _debug_vprintf(const char *format, va_list ap)
 #elif defined(PIPE_SUBSYSTEM_WINDOWS_MINIPORT)
    /* TODO */
 #else /* !PIPE_SUBSYSTEM_WINDOWS */
-#ifdef DEBUG
    fflush(stdout);
    vfprintf(stderr, format, ap);
 #endif
-#endif
 }