drm/amdgpu: use GEM references instead of TTMs v2

Instead of a TTM reference grab a GEM reference whenever necessary.

v2: fix typo in amdgpu_bo_unref pointed out by Vitaly,
    initialize the GEM funcs for kernel allocations as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>