drm/amd/display: revert order change of HUBP and MPC disable

- root cause was we disable opp clk in MPC disconnect
- hubp_blank is not double buffered, so we can't blank until MPC disconnect or we have risk of underflow

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>