drm/amd/display: Extend DMCUB offload testing into dcn20/21

drm/amd/display: Extend DMCUB offload testing into dcn20/21

[Why]
To quickly validate whether DMCUB is running and accepting commands for
offload testing we want to intercept a common sequence as part of
modeset programming.

[How]
OTG enable will cause the most impact in terms of golden register
changes and it's a single register write.

This approach was previously done in dcn10 code when it was shared with
dcn20 but it wasn't ported over to the dcn20 code.

Port over start, execute and wait sequence into dcn20_optc.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@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>