MGS-4569 [#imx-1347] fix GPU hang when play 4K video

kernel process(with zero pid) has no mdl mapping,
skip kernel process to avoid invalid mdl access.

also remove memory barrier to avoid rcu issue.

Signed-off-by: Xianzhong <xianzhong.li@nxp.com>

drivers/mxc/gpu-viv/hal/os/linux/kernel/gc_hal_kernel_os.c

@@ -4500,7 +4500,7 @@ _CacheOperation(
     )
 {
     PLINUX_MDL mdl = (PLINUX_MDL)Handle;
-    PLINUX_MDL_MAP mdlMap;
+    PLINUX_MDL_MAP mdlMap = gcvNULL;
     gckALLOCATOR allocator;
 
     if (!mdl || !mdl->allocator)
@@ -4513,11 +4513,14 @@ _CacheOperation(
 
     if (allocator->ops->Cache)
     {
-        mutex_lock(&mdl->mapsMutex);
+        if (ProcessID)
+        {
+            mutex_lock(&mdl->mapsMutex);
 
-        mdlMap = FindMdlMap(mdl, ProcessID);
+            mdlMap = FindMdlMap(mdl, ProcessID);
 
-        mutex_unlock(&mdl->mapsMutex);
+            mutex_unlock(&mdl->mapsMutex);
+        }
 
         if (ProcessID && mdlMap == gcvNULL)
         {
@@ -4534,8 +4537,6 @@ _CacheOperation(
         }
     }
 
-    _MemoryBarrier();
-
     return gcvSTATUS_OK;
 }