drm/amdgpu: add amdgpu_ttm_buffer_entity_fini func

drm/amdgpu: add amdgpu_ttm_buffer_entity_fini func

This allows to have init/fini functions to hold all the init and
teardown code for amdgpu_ttm_buffer_entity.
For now only drm_sched_entity init/destroy function calls are moved
here, but as entities gain new members it will make code simpler.

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>