drm/ttm: make ttm_pool.h self-contained

struct seq_file needs a forward declaration in some configs. Sort the
forward declarations while at it.

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403072259.EEC2Vf1X-lkp@intel.com/


Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b92373759bba305ddf8d24fdca345f195400e206.1709898638.git.jani.nikula@intel.com