drm/amdgpu: move gem definitions into amdgpu_gem header

Demangle amdgpu.h.

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>