drm/amdgpu/soc15:Add vega20 soc15_common_early_init support

Set external_rev_id and disable cg,pg for now.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NFeifei Xu <Feifei.Xu@amd.com>
Reviewed-by: NHawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: NHuang Rui <ray.huang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/soc15.c

@@ -658,6 +658,11 @@ static int soc15_common_early_init(void *handle)
 		adev->pg_flags = 0;
 		adev->external_rev_id = adev->rev_id + 0x14;
 		break;
+	case CHIP_VEGA20:
+		adev->cg_flags = 0;
+		adev->pg_flags = 0;
+		adev->external_rev_id = adev->rev_id + 0x28;
+		break;
 	case CHIP_RAVEN:
 		adev->cg_flags = AMD_CG_SUPPORT_GFX_MGCG |
 			AMD_CG_SUPPORT_GFX_MGLS |