drm/amd/display: Fixed using wrong eDP power sequence function pointer

[why]
dc->hwss->edp_backlight_control is null, it would casue it only be off
main-link of eDP.  It is not worng behavior for eDP power sequence off.

[how]
Must use hwseq->funcs.edp_backlight_control finction pointer for edp
backlight.

Signed-off-by: Yi-Ling Chen <Yi-Ling.Chen2@amd.com>
Reviewed-by: Sung Lee <Sung.Lee@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>