drm/amd: the data retured from PRT is expected to be 0

The dummy page for returning from PRT resides inside system memory,
need set system flag bit in VM_L2_CNTL.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>