[Why]

DCN IPS interoperates with other system idle power features, such as
Zstates.

On DCN35, there is a known issue where system Z8 + DCN IPS2 causes a
hard hang. We observe this on systems where the SBIOS allows Z8.

Though there is a SBIOS fix, there's no guarantee that users will get it
any time soon, or even install it. A workaround is needed to prevent
this from rearing its head in the wild.

[How]

For DCN35, check the pmfw version to determine whether the SBIOS has the
fix. If not, set IPS1+RCG as the deepest possible state in all cases
except for s0ix and display off (DPMS). Otherwise, enable all IPS
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 28d43d08)
Cc: stable@vger.kernel.org