drm/amd/display: Fix a logic defect in cursor move

Regression caused by: Ib98354194d7

Need to check crtc->stream before updating cursor attributes
and position.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>