drm/amdgpu: Enable async flip on overlay planes

amdgpu can handle async flips on overlay planes, so allow it for atomic
async checks.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250127-tonyk-async_flip-v12-2-0f7f8a8610d3@igalia.com


[DB: fixed checkpatch warning by adding braces]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>