vt: do not update when the console is blanked

vt.c DO_UPDATE macro checks if the console is visible but doesn't check if
the console is blanked.

In fact updating fbcon while the console is blanked is not only
unnecessary but can even cause screen corruption.

Therefore I am adding a simple check on console_blanked in DO_UPDATE.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>