drm/amdgpu/display: add support for multiple backlights

On platforms that support multiple backlights, register
each one separately.  This lets us manage them independently
rather than registering a single backlight and applying the
same settings to both.

v2: fix typo:
Reported-by: kernel test robot <lkp@intel.com>

Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>