vt(4): Skip vt_window_switch() for nested panics

[Why]
The same protection was added to vt_flush() in the previous commit. We
want the same one in vt_window_switch(): if e.g. the DRM driver panics
while handling a call to vt_window_switch(), we don't want to
recursively call vt_window_switch() again and trigger another panic.

Reviewed by: imp, manu
Approved by: imp, manu
Differential Revision: https://reviews.freebsd.org/D42750

(cherry picked from commit 24d6f256f825e5d7f8ca6b5d16499f13614f9cdd)