drm/ttm: add some kernel-doc for TTM_TT_FLAG_*

Move it to inline kernel-doc, otherwise we can't add empty lines it
seems. Also drop the kernel-doc for pages_list, which doesn't seem to
exist.

v2(Christian):
  - Add a note that FLAG_SWAPPED shouldn't need to be touched by drivers.
  - Mention what FLAG_POPULATED does.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929132629.353541-2-matthew.auld@intel.com
Signed-off-by: Christian König <christian.koenig@amd.com>