drm/amd/display: Get backlight controller id from link

drm/amd/display: Get backlight controller id from link

[Why]
dc_link_set_backlight_level can be called from a context
where the stream is unknown.  In this case, we can still
find which controller is driving this particular backlight

[How]
Compare links for equality instead of streams

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>