drm/amdgpu: remove adev pointer from struct amdgpu_bo v2

It's completely pointless to have two pointers to the
device in the same structure.

v2: rename function to amdgpu_ttm_adev, fix typos

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