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

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

[How]
SR(BIOS_SCRATCH_2) to NBIO_SR(BIOS_SCRATCH_2).

Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: <stable@vger.kernel.org>