mrsas: don't reference the removed physical disk of RAID1 during IO submission

When a physical disk(PD) [belonging to a RAID1 Virtual disk(VD)] is
removed, driver may still use the reference to the removed PD while submitting
IO to the controller. Controller firmware faults upon receipt of such IO.
This patch fixes this issue by not using any reference to the removed PD.

Reviewed by: imp
Approved by: imp
Sponsored by: Broadcom Inc
Differential Revision: https://reviews.freebsd.org/D44282