cortex_m: Avoid unnecessary saving and restoring of DCRDR

This is used for the emulated DCC channel which is only maintained as long
as target->dbg_msg_enabled is set. Skip the saving and restoring if not
enabled to save one dap_run() per core register access.

Note that we could've probably queued all core register accesses in the
same transaction if the armv7 register framework hadn't required
synchronous register accesses.

Change-Id: I4fe6d713261ee5db42422203eb63035fdcc48891
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1848


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>