intelfb: fix a bug when changing video timing

When changing video timing dynamically via fbset the screen sporadically
is rendered black.

With the attached fix which disables VCO prior to timing register change
the problem disappears.

I had a look at the Xserver register setup code. Here the VCO is
disabled in the same way [1].

This patch is taken from vga-sync-field version 0.0.11 [2][3].

[1] http://cgit.freedesktop.org/xorg/driver/xf86-video-intel/tree/src/i830_=
driver.c
[2] http://lowbyte.de/vga-sync-fields/vga-sync-fields-0.0.11.tgz
[3] http://easy-vdr.de/git?p=frc.git/.git;a=commit;h=dcc3b863e5a663652587619c357bd20075af6896
2587619c357bd20075af6896

Signed-off-by: Thomas Hilber <sparkie@lowbyte.de>
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>