drm/amd/display: Skip vendor specific LTTPR w/a outside link training

[Why]
Vendor specific link rate toggle workaround should not be
applied outside of link training scenario, such as when
outputting PHY test pattern.

[How]
Add flag to indicate when link training is occuring.
Make workaround a no-op when flag is not set.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: George Shen <George.Shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>