[Why]
Backlight adjustment is tied to a specific display.  So make the calls
target a link rather than making it a global state.

[How]
make all backlight calls link based
Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>