drm/amd/display: Fix incorrect backlight register offset for DCN

[Why]
Typo in backlight refactor inctroduced wrong register offset.

[How]
Change DCE to DCN register map for PWRSEQ_REF_DIV

Cc: stable@vger.kernel.org
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>