drm/ttm: clean up non-x86 definitions on ttm_page_alloc_dma

All non-x86 definitions are moved to ttm_set_memory header, so remove it from
ttm_page_alloc_dma.c.

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>