drm/amd/display: Set mpcc_disconnect_pending during MPC reset

This prevents an issue where the MPCC will not go to idle due to us not
waiting for it to become idle during disable plane.

Signed-off-by: Andrew Jiang <Andrew.Jiang@amd.com>
Signed-off-by: Tony Cheng <tony.cheng@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>