agp/intel: the GMCH is always enabled for integrated processor graphics

... and trying to set the bit is ineffectual.

Fixes the regression from e380f60b which detected that we were trying to
do undefined operations on the I830_GMCH_CTRL.

Reported-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>