drm/ttm: add ttm_set_memory header (v2)

This patch moves all non-x86 abstraction to the ttm_set_memory header.
It is to make function calling more clearly.

(v2): add ttm_ prefix.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Bas Nieuwenhuizen <basni@chromium.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>