drm/amd/display/amdgpu_dm: Fix backlight_device_register() error handling

backlight_device_register() returns an ERR_PTR on error, but other code
such as amdgpu_dm_connector_destroy() assumes dm->backlight_dev[i] is NULL
if no backlight is registered.

Clear dm->backlight_dev[i] on registration failure, to avoid other code
trying to deref an ERR_PTR pointer.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>