drm/ttm: s/FLAG_SG/FLAG_EXTERNAL/

It covers more than just ttm_bo_type_sg usage, like with say dma-buf,
since one other user is userptr in amdgpu, and in the future we might
have some more. Hence EXTERNAL is likely a more suitable name.

v2(Christian):
  - Rename these to TTM_TT_FLAGS_*
  - Fix up all the holes in the flag values

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929132629.353541-1-matthew.auld@intel.com
Signed-off-by: Christian König <christian.koenig@amd.com>