[Why]
This hook that is supposed to read the actual backlight value
is used in a few places throughout the kernel to setup or force
update on backlight

[How]
Create a dc function that calls the existing abm function, and
call that function from amdgpu
Signed-off-by: NDavid Francis <David.Francis@amd.com>
Reviewed-by: NHarry Wentland <Harry.Wentland@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>