ide: set REQ_PREEMPT request flag in ide_do_drive_cmd() users

* Set REQ_PREEMPT request flag in ide_do_drive_cmd() users
  for ide_preempt and ide_head_wait action types.

* Remove setting REQ_PREEMPT from ide_do_drive_cmd().

While at it:

* Set 'where' variable outside ide_lock.

This is a preparation for converting IDE to use blk_execute_rq().

There should be no functional changes caused by this patch.

Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Borislav Petkov <petkovbb@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>