drm/amd/display: Null ptr check for set_sdr_white_level

[Why&How]
Cursor boosting can only be done on DCN+
Check for nullptr since DCE doesn't implement it.

Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Reviewed-by: Sivapiriyan Kumarasamy <Sivapiriyan.Kumarasamy@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>