drm/amd/display: Early return when turn off a plane.

In case of two monitor connected and turn off one of the monitors,
OTG0 is locked after graphic plane off due to redundant programming
front end regs.

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>