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
09855acb1c2e3779f25317ec9a8ffe1b1784a4a8

Argument @sg was added by Dave in commit
129b78bfca591e736e56a294f0e357d73d938f7e

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>