os: NextDPMSTimeout: mark intentional fallthroughs in switch

The comment at the top of the function tells humans the fallthroughs
are intentional, but gcc doesn't parse that.

Clears 3 -Wimplicit-fallthrough warnings from gcc 14.1

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1716>
(cherry picked from commit b306df5a60)

os/WaitFor.c

@@ -418,13 +418,13 @@ NextDPMSTimeout(INT32 timeout)
     switch (DPMSPowerLevel) {
     case DPMSModeOn:
         DPMS_CHECK_TIMEOUT(DPMSStandbyTime)
-
+        /* fallthrough */
     case DPMSModeStandby:
         DPMS_CHECK_TIMEOUT(DPMSSuspendTime)
-
+        /* fallthrough */
     case DPMSModeSuspend:
         DPMS_CHECK_TIMEOUT(DPMSOffTime)
-
+        /* fallthrough */
     default:                   /* DPMSModeOff */
         return 0;
     }