drm/amd/amdgpu: add check that shadow page tables are GPU-accessible

Skip amdgpu_gem_va_update_vm otherwise. Also clean up the check for the
non-shadow page tables using the new helper function.

This fixes a crash with the stack trace:

amdgpu_gem_va_update_vm
-> amdgpu_vm_update_page_directory
 -> amdgpu_ttm_bind
  -> amdgpu_gtt_mgr_alloc

v2: actually check bo->shadow instead of just checking bo twice

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