drm/amdgpu: check the GART table before invalidating TLB

Bypass group programming (utcl2_harvest) aims to forbid UTCL2 to send
invalidation command to harvested SE/SA. Once invalidation command comes
into harvested SE/SA, SE/SA has no response and system hang.

This patch is to add checking if the GART table is already allocated before
invalidating TLB. The new procedure is as following:
1. Calling amdgpu_gtt_mgr_init() in amdgpu_ttm_init(). After this step GTT
   BOs can be allocated, but GART mappings are still ignored.
2. Calling amdgpu_gart_table_vram_alloc() from the GMC code. This allocates
   the GART backing store.
3. Initializing the hardware, and programming the backing store into VMID0
   for all VMHUBs.
4. Calling amdgpu_gtt_mgr_recover() to make sure the table is updated with
   the GTT allocations done before it was allocated.
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -259,6 +259,9 @@ void amdgpu_gart_invalidate_tlb(struct amdgpu_device *adev)
 {
 	int i;
 
+	if (!adev->gart.ptr)
+		return;
+
 	mb();
 	amdgpu_device_flush_hdp(adev, NULL);
 	for (i = 0; i < adev->num_vmhubs; i++)