drm/amd/display: Fix static checker warnings on tracebuff_fb

[Why]
Static analysis on linux-next has found a potential null pointer
dereference;

[How]
Refactor the function, add ASSERT and remove the unnecessary check.

Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>