drm/amdgpu: fix and cleanup VM ready check

Stop checking the mapped BO itself, cause that one is
certainly not a page table.

Additional to that move the code into amdgpu_vm.c

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>