drm/amd/display: fix application of sizeof to pointer

Both of split and merge are pointers, not arrays.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>