drm/i915: Add pipe B force quirk for 830M

830M has problems when some of the pipes are disabled. Namely if a
plane, DVO port etc. is currently assigned to a disabled pipe, it
can't moved to the other pipe until the current pipe is also enabled.
To keep things simple just leave both pipes running all the time.

Ideally I think should turn the pipes off if neither is active, and
when either becomes active we enable both. But that would reuquire
proper atomic modeset support, and probably a bit of extra care in
the order things get enabled.

v2: Reorder wrt. double wide handling changes

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>