drm/amdgpu: add macro to retrieve timeline name v2

This helps de-duplicate a long expression and removes overly long lines.

v2: Rename macro and undef it

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>