drm/amdgpu: remove begin_job/finish_job

Completely pointless and confusing to use a callback
to call into the same code file.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Monk.Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>