drm/amd/display: Check null pointer before dereferencing se

[WHAT & HOW]
se is null checked previously in the same function, indicating
it might be null; therefore, it must be checked when used again.

This fixes 1 FORWARD_NULL issue reported by Coverity.

Acked-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>