drm/amd/display: Fix uninitialized variable.

[Why]
Uninitialized variable causes diag compilation build failure.

[How]
- Ensure that variable in question is always initialized before being
used.
- The variable in question is the USB4 DP training pattern. In case an
unsupported training pattern has been requested, update status
accordingly and abort current link training attempt.

Reviewed-by: Meenakshikumar Somasundaram <Meenakshikumar.Somasundaram@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>