savagefb: fix blanking mode on CRT display

Fix wrong bit mask for blanking register.  Due to the error a CRT monitor
blanks off due to wrong frequency (out of range) instead of PM signal
(vertical and horizontal frequencies cut off).

Just compare the mask with bits set in the switch(blank) clause below the
changed line.

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