drm/amdgpu: increase reserved VRAM size to 8MB

4MB reserved VRAM size which used for page tables was not enough for
some condition, increase it to 8MB to reduce page table contention.
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@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_vm.h

@@ -107,7 +107,7 @@ struct amdgpu_bo_list_entry;
 #define AMDGPU_VM_FAULT_STOP_ALWAYS	2
 
 /* Reserve 4MB VRAM for page tables */
-#define AMDGPU_VM_RESERVED_VRAM		(4ULL << 20)
+#define AMDGPU_VM_RESERVED_VRAM		(8ULL << 20)
 
 /* max number of VMHUB */
 #define AMDGPU_MAX_VMHUBS			3