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 <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Acked-by: Alex Hung <alex.hung@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>