drm/amdgpu: Fix double deletion of validate_list

drm/amdgpu: Fix double deletion of validate_list

If amdgpu_amdkfd_gpuvm_free_memory_of_gpu() fails after kgd_mem is
removed from validate_list, the mem handle still lingers in the KFD idr.
This means when process is terminated,
kfd_process_free_outstanding_kfd_bos() will call
amdgpu_amdkfd_gpuvm_free_memory_of_gpu() again resulting in double
deletion.

To avoid this -
 (a) Check if list is empty before deleting it
 (b) Rearragne amdgpu_amdkfd_gpuvm_free_memory_of_gpu() such that it can
     be safely called again if it returns failure the first time.

Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>