monitor_reconfig: Disable external output to avoid error.

monitor_reconfig: Disable external output to avoid error.

If we're using 1920x1200 on an external output and then
unplug it and try to switch back to 1366x768, xrandr gives
us an error like:

xrandr: specified screen 1366x768 not large enough for
output VGA1 (1920x1200+0+0)

when trying to set the screen size.  I think that this is
harmless, but it's easy enough to disable the external
output first in this case before adjusting the screen size.

Change-Id: I344f5af66657cb935b33d4398ec8a94befcf2ead

BUG=none
TEST=plugged and unplugged VGA and ran monitor_reconfigure a bunch

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=a233cdb

[sosa, 2010-09-17 17:41:37]

LGTM