drm/amdgpu: add amdgpu.h (v2)

This is the main header file for amdgpu.

v2: remove stable comments
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>