drm/amdkcl: [4.8] fix Centos7.4 modprobe hang problem

[Why]
adev->ddev->mode_config.helper_private =
&amdgpu_dm_mode_config_helperfuncs;
will not be used when using 4.12 as drm control version.
So the hang problem happens.

[How]
Using 4.8 as drm control version according to old kcl patch:
drm/amdkcl: [RHEL 6] Include kernel.h in chash.c for pr_fmt definition
The use of helper_private can be traced to patch:
drm/atomic-helper: nonblocking commit support
This patch is introduced in drm version(4.8.0).
So we need to use 4.8 as drm control version.

Change-Id: I19c57332c8f744e4edccc11a6d5758bbc20f1cc5
Signed-off-by: changzhu <Changfeng.Zhu@amd.com>
Reviewed-by: Tianci Yin <tianci.yin@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1782,7 +1782,7 @@ static int amdgpu_dm_mode_config_init(struct amdgpu_device *adev)
 	adev->mode_info.mode_config_initialized = true;
 
 	adev->ddev->mode_config.funcs = (void *)&amdgpu_dm_mode_funcs;
-#if DRM_VERSION_CODE >= DRM_VERSION(4, 12, 0)
+#if DRM_VERSION_CODE >= DRM_VERSION(4, 8, 0)
 	adev->ddev->mode_config.helper_private = &amdgpu_dm_mode_config_helperfuncs;
 #endif