drm/ttm: remove the init_mem_type callback

It is a very strange concept to call a function which just
calls back the caller for the functions parameters.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/382085/