drm/amd/display: Don't check registers, if using AUX BL control

[Why]
Currently the driver looks DCN registers to access if BL is on or not.
This check is not valid if we are using AUX based brightness control.
This causes driver to not send out "backlight off" command during power off
sequence as it already thinks it is off.

[How]
Only check DCN registers if we aren't using AUX based brightness control.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Swapnil Patel <swapnil.patel@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>