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