drm/amd/display: fix dprefclk and ss percentage reading on RN

[Why]
Before was using HW counter value to determine the dprefclk. Which
take into account ss, but has large variation, not good enough for
generating audio dto. Also, the bios parser code to get the ss
percentage was not working.

[How]
After this change, dprefclk is hard coded, same as on RV. We don't
expect this to change on Renoir. Modified bios parser code to get
the right ss percentage.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>