mem: Print an MSHR without triggering any assertions

mem: Print an MSHR without triggering any assertions

Previously printing an mshr would trigger an assertion if the MSHR was
not in service or if the targets list was empty. This patch changes
the print function to bypasses the accessor functions for
postInvalidate and postDowngrade and avoid the relevant assertions. It
also checks if the targets list is empty before calling print on it.

Change-Id: Ic18bee6cb088f63976112eba40e89501237cfe62
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>