drm/amd/display: Fix use of uninitialized union

[Why]
An uninitialized variable would randomly initialize to a large
value. This caused enough delay to fail DP Compliance Test 400.2.1.

[How]
Initialize the variable.

Signed-off-by: John Barberiz <John.Barberiz@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>