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 <[e-mail]>
Cc: Huang Rui <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Reviewed-by: Christian König <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/b92[e-mail]

include/drm/ttm/ttm_pool.h

···
 #include <drm/ttm/ttm_caching.h>
 
 struct device;
-struct ttm_tt;
-struct ttm_pool;
+struct seq_file;
 struct ttm_operation_ctx;
+struct ttm_pool;
+struct ttm_tt;
 
 /**
  * struct ttm_pool_type - Pool for a certain memory type