drm/ttm: remove nonsense AGP handling

map_page_into_agp() and unmap_page_from_agp() are only defined on x86.

On all other platforms they are defined as noops. So this code doesn't has any effect at all.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/391292/