drm/radeon/mkregtable: Delete unused list functions and macros

The util mkregtable includes a copy of the kernel API for linked lists,
only a small subset of it is used. Delete the unused functions and macros.
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>