vt(4): Make sure vt_switch_timer() has a sleepable context.

Fixes the following panic backtrace:

panic()
usbhid_sync_xfer()
usbhid_set_report()
hid_set_report()
hidbus_write()
hid_write()
hkbd_set_leds()
hkbd_ioctl_locked()
hkbd_ioctl_locked()
hkbd_ioctl()
kbdmux_ioctl()
vt_window_switch()
vt_switch_timer()

Differential Revision: https://reviews.freebsd.org/D36715
MFC after: 1 week
Sponsored by: NVIDIA Networking