tests/kms_plane_multiple: turn off pipe if all planes are off

amdgpu rejects when crtc is on + all planes are off, and it
is necessary to turn off crtc when all planes are off in the
subtest "tiling-none".

This is revised from https://patchwork.freedesktop.org/series/80904/



Signed-off-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>