[WHY]
If DCN is in idle then we should not be accessing DCN registers or
it can lead to hangs.

[HOW]
Log the error and return 0 or drop the write if it's in idle.

This is skipped in the exit sequence itself since the boolean flips
before it starts.

It also does not cover accesses from external clients outside of DM/DC
like firmware or the kernel mode driver.
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>