drm/amd/display: Remove redundant condition in dml vba

Found by 0-day
https://lists.freedesktop.org/archives/dri-devel/2017-October/154698.html



Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>