a) Embed struct drm_device into struct amdgpu_device.
b) Modify the inline-f drm_to_adev() accordingly.
c) Modify the inline-f adev_to_drm() accordingly.
d) Eliminate the use of drm_device.dev_private,
   in amdgpu.
e) Switch from using drm_dev_alloc() to
   drm_dev_init().
f) Add a DRM driver release function, which frees
   the container amdgpu_device after all krefs on
   the contained drm_device have been released.

v2: Split out adding adev_to_drm() into its own
    patch (previous commit), making this patch
    more succinct and clear. More detailed commit
    description.
v3: squash in fix to call drmm_add_final_kfree()
    to avoid a warning.
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>