drm/amdgpu: Fix BUG_ON assert

With added CPU domain to placement you can have
now 3 placemnts at once.

CC: stable@kernel.org
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210622162339.761651-5-andrey.grodzovsky@amd.com