drm/ttm: cleanup ttm_agp_backend

Audit the includes and stop accessing the internal drm_mm_node.

The ttm_resource::start is the same value as the drm_mm_node::start.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210426175824.122557-1-christian.koenig@amd.com