drm/amd/display: Add W/A for PHY tests with certain LTTPR

[Why]
Certain LTTPR require output VS/PE to be explicitly
set during PHY test automation.

[How]
Add vendor-specific sequence to set LTTPR
output VS/PE.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: George Shen <George.Shen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>