drm/ttm_bo_api: update the description for @placement and @sg

drm/ttm_bo_api: update the description for @placement and @sg

Correct the argument name of @placement and added @sg description for
ttm_bo_init() and ttm_bo_init_reserved().

Argument @flags was replaced to @placement by Jerome in commit
4b3b1d095cd497d0449ce8e4f482efc0db267612

Argument @sg was added by Dave in commit
41cd9ab871f163d54dae52c5cfea34bf4e74a277

Signed-off-by: Amos Kong <amos@sietium.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/38eac09bf2ddd6088cc8a126e6bc4792eaa2dc88.1633462176.git.amos@sietium.com
Signed-off-by: Christian König <christian.koenig@amd.com>