vt: drop the lock before calling vd_postswitch

[kevans]

Summary

Some postswitch methods, e.g., from i915kms, don't currently expect to
be called under a lock and may try to sleep. The expectation seems OK,
we usually do drop the lock prior to calling vd_postswitch, so let's
avoid an unnecessary panic and drop the lock earlier..

[jhb]

sys/dev/vt/vt_core.c

625

Notably, we drop the VT lock before postswitch here already. Might be worth calling out that you are fixing a special case to follow the same behavior as the more general case. Probably the special cal to vd_postswitch was added later and that later commit just got the locking wrong.