drm/admgpu: add two shadow BO helper functions

Add amdgpu_bo_add_to_shadow_list() to handle shadow list
additions and amdgpu_bo_shadowed() to check if a BO is shadowed.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>