fbcon: prevent cursor disappearance after switching to 512 character font

Adjust and honor the vc_scrl_erase_char for 256 and 512 character fonts.

It fixes the issue with disappearing cursor during scrolling
(http://bugzilla.kernel.org/show_bug.cgi?id=11258).  The issue was
reported and tracked by Peter Hanzel.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Reported-by: Peter Hanzel <hanzelpeter@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>