drm/amd/display: turn off cursor when disconnect plane

As a precaution to prevent cases where cursor is enabled on a pipe
that is disabled, always turn off cursor when disconnecting plane.

Signed-off-by: Eric Yang <Eric.Yang2@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>