pause_sbt(): Take the cold path (ie. use DELAY()) if KDB is active

This fixes a panic in the i915 driver when one uses debug.kdb.enter=1
under vt(4).

PR:		193269
Reported by:	emaste@
Submitted by:	avg@
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271253 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f