RDMA/rxe: Move code to check if drained to subroutine

Move two blocks of code in rxe_comp.c and rxe_req.c to subroutines that
check if draining is complete in the SQD state and, if so, generate a
SQ_DRAINED event.

Link: https://lore.kernel.org/r/20230405042611.6467-4-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>