drm/amd/display: Remove stream_res tg null check in commit planes

[Why]
TG in pipe_ctx stream resource is not null where used in
commit_planes_do_stream_update since it is assigned the
same time the stream is set in pipe_ctx - when the pipe is
acquired. This null check produced a static code analysis
warning and should be removed to avoid similar unnecessary
checks in the future.

[How]
Remove the null check.

Signed-off-by: SivapiriyanKumarasamy <sivapiriyan.kumarasamy@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>