drm/amd/display: Cache cursor position.

drm/amd/display: Cache cursor position.

When programming cursor position after front end programmed,
if position is already set previously, it doesn't make sense
to program position with all 0.
Cache position and use the parameter after front end programming.

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>