drm/amdgpu: fix parentheses in amdgpu_vm_update_ptes

For the root PD mask can be 0xffffffff as well which would
overrun to 0 if we don't cast it before we add one.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>