drm/amdgpu: move sdma definitions into amdgpu_sdma header

Demangle amdgpu.h.
Furthermore, SDMA is used for moving and clearing the data buffer, so the header
also need be included in ttm.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>