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>

drivers/gpu/drm/amd/display/dc/core/dc_stream.c

@@ -214,6 +214,7 @@ bool dc_stream_set_cursor_attributes(
 		}
 
 		core_dc->hwss.set_cursor_attribute(pipe_ctx);
+		if (core_dc->hwss.set_cursor_sdr_white_level)
 			core_dc->hwss.set_cursor_sdr_white_level(pipe_ctx);
 	}