drm/amd/display: Don't allow IPS2 in D0 for RCG Dynamic

[Why]
The existing changes to the DPMS off flag should help reduce
accidental entry, but this change further restricts the entry condition.

[How]
Record last power state as sent to DMUB.
Don't send IPS2 allow if it's D0.

Reviewed-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>