drm/amd/display: Notify idle link detection through shared state

[Why]
We can hang in IPS2 checking DMCUB_SCRATCH0 for link detection state.

[How]
Replace the HW access with a check on the shared state bit. This will
work the same way as the SCRATCH0 but won't require a wake in the case
where link detection isn't required.

Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>