drm/amd/display: Add shared firmware state for DMUB IPS handshake

[Why]
Read modify write hazards can occur when using a single shared scratch
register between driver and firmware leading to driver accessing DCN
in IPS2 and a system hang.

[How]
Add infrastructure for using REGION6 as a shared firmware state between
driver and firmware. This region is uncachable.

Replace the existing get/set idle calls with reads/writes to the
(volatile) shared firmware state blocks that a separated by at least
a cache line between firmware and driver.

Remove the workarounds that required rewriting/checking read modify
write hazards.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@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>