drm/amd/display: Fix idle check for shared firmware state

drm/amd/display: Fix idle check for shared firmware state

[WHY]
We still had an instance of get_idle_state checking the PMFW scratch
register instead of the actual idle allow signal.

[HOW]
Replace it with the SW state check for whether we had allowed idle
through notify_idle.

Cc: Mario Limonciello <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: [email protected]
Reviewed-by: Duncan Ma <[email protected]>
Acked-by: Alex Hung <[email protected]>
Signed-off-by: Nicholas Kazlauskas <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>