drm/amdgpu: drop legacy drm load and unload callbacks

We've moved the debugfs handling into a centralized place
so we can remove the legacy load an unload callbacks.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>