drm/amd/display: Change power gating off sequence to fix hang

Power off plane clear all the reg values includes cursor.
When OS call set cursor position, cursor address reg is cleared,
results in system hard hang.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>