drm/amd/display: Initialize stream_update to zero

[Why]
The stream_update struct is left unitialized but DC will access
its fields. This usually results in global state validation occur
during any atomic commit with state->allow_modeset = true.

[How]
Initialize the struct to zero for every stream we check.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>